U.S. patent application number 15/968421 was filed with the patent office on 2019-11-07 for differential termination modulation for back-channel communication.
The applicant listed for this patent is Euhan Chong. Invention is credited to Euhan Chong.
Application Number | 20190341963 15/968421 |
Document ID | / |
Family ID | 68383582 |
Filed Date | 2019-11-07 |
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United States Patent
Application |
20190341963 |
Kind Code |
A1 |
Chong; Euhan |
November 7, 2019 |
DIFFERENTIAL TERMINATION MODULATION FOR BACK-CHANNEL
COMMUNICATION
Abstract
Devices and methods for communicating back-channel data over a
communication link are provided. The termination impedance of the
communication link at the receiver and/or transmitter side may be
modulated to encode back-channel data as signal reflections in the
communication link. The corresponding device at the other end of
the communication link may detect these reflections and decode them
to recover the back-channel data.
Inventors: |
Chong; Euhan; (Ottawa,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Chong; Euhan |
Ottawa |
|
CA |
|
|
Family ID: |
68383582 |
Appl. No.: |
15/968421 |
Filed: |
May 1, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04L 1/208 20130101;
H04B 3/18 20130101 |
International
Class: |
H04B 3/18 20060101
H04B003/18; H04L 1/20 20060101 H04L001/20 |
Claims
1. A receiver for receiving a data signal over a communication link
and sending back-channel data over the communication link,
comprising: a first resistive element having an adjustable first
resistance, the first resistance being adjusted based on a received
first resistor tuning signal; a second resistive element having a
second resistance; a terminator for differentially terminating the
communication link using the first resistive element and the second
resistive element; and a back-channel data encoder for: receiving a
back-channel data signal; and providing a first resistor tuning
signal to the first resistive element based on the back-channel
data signal.
2. The receiver of claim 1 wherein: the second resistance is
adjustable and is adjusted based on a received second resistor
tuning signal; and the back-channel data encoder is further
configured to provide a second resistor tuning signal to the second
resistive element based on the back-channel data signal.
3. The receiver of claim 2 wherein, to encode a bit of the
back-channel data, the back-channel data encoder provides: a first
resistor tuning signal that causes the first resistance to change
in either a positive or negative direction; and a second resistor
tuning signal that causes the second resistance to change in the
same direction as the first resistance.
4. The receiver of claim 3 wherein the change in the first
resistance is equal to the change in the second resistance.
5. The receiver of claim 3 wherein the change in the first
resistance is different from the change in the second
resistance.
6. The receiver of claim 2 wherein, to encode a bit of the
back-channel data, the back-channel data encoder provides: a first
resistor tuning signal that causes the first resistance to change
in either a positive or negative direction; and a second resistor
tuning signal that causes the second resistance to change in the
opposite direction from the first resistance.
7. The receiver of claim 6 wherein the absolute value of the change
in the first resistance is equal to the absolute value of the
change in the second resistance.
8. The receiver of claim 7 wherein: the first resistance has a
first baseline value; and the first resistance stays within ten
ohms of the first baseline value while being changed by the first
resistor tuning signal.
9. The receiver of claim 7 wherein: the first resistance has a
first baseline value; and the absolute value of the difference
between the first resistance and the first baseline value stays
below twenty percent of the first baseline value while the first
resistance is being changed by the first resistor tuning
signal.
10. The receiver of claim 9 wherein: the second resistance has a
second baseline value; and the absolute value of the difference
between the second resistance and the second baseline value stays
below ten percent of the second baseline value while the second
resistance is being changed by the second resistor tuning
signal.
11. The receiver of claim 2 wherein the first resistive element
comprises a plurality of parallel resistor slices, one or more of
which are activated or deactivated based on the first resistor
tuning signal.
12. The receiver of claim 2 wherein: the data signal received over
the communication link has a unit interval of time corresponding to
a clock cycle; and the back-channel data encoder changes the value
of the first resistor tuning signal less often than once every ten
unit intervals.
13. The receiver of claim 2 wherein the receiver comprises a
serializer-deserializer (SerDes) receiver.
14-26. (canceled)
27. The receiver of claim 2 wherein the back-channel data encoder
comprises: a memory containing instructions for encoding the
back-channel data as signal reflections in the communication link
by providing the first resistor tuning signal and the second
resistor tuning signal to modulate the first resistance and second
resistance respectively; and a processor configured to execute the
instructions stored in the memory.
28. The receiver of claim 2 wherein the back-channel data encoder
comprises an integrated circuit configured to encode the
back-channel data as signal reflections in the communication link
by providing the first resistor tuning signal and the second
resistor tuning signal to modulate the first resistance and second
resistance respectively.
Description
FIELD
[0001] The present disclosure relates to a method and apparatus for
the transmission of data over a low bandwidth link from a received
towards a transmitter. In particular, the present disclosure
relates to devices and methods for enabling a backchannel
communication link between a receiver and the corresponding
transmitter on a point to point serial transmission link.
BACKGROUND
[0002] Serial data links are used for communication between a
transmitter and a receiver. Systems using serial data links often
include serializer-deserializers (SerDes, or SERDES), which consist
of a pair of functional blocks, one at each of the transmitter and
receiver, that are used for high-speed communication between two
nodes, such as two application-specific integrated circuits
(ASICs), across a limited input/output link between the two
nodes.
[0003] Some nodes will include at least one transmitter and at
least one receiver, thereby allowing bidirectional communication,
although some such communication systems will use only transmitters
on the first node and only receivers on the second node. In any
case, serial data links are traditionally designed with standalone
transmitter (TX) and receiver (RX) sides.
[0004] For a serial data link to operate most efficiently, it is
desirable for the TX and RX ends of the link to be able to share
performance-related information. However, most systems do not have
an inherent ability to communicate this information between the
transmitter and receiver or vice-versa. Serial data links
communicate high-speed data from node to node (e.g. ASIC to ASIC),
but are not able to add overhead data to live bit streams, so the
performance-related information cannot be encoded in the data
stream. Therefore, it is not possible to communicate
performance-related information over a serial data link when the
data link is active.
[0005] One known solution to this problem is to use dedicated
circuitry, pins and physical wire connections to create a dedicated
physical backchannel for communication of performance-related
information or other metadata from the receiver back towards the
transmitter of the serial connection. However, this is a large,
undesirable overhead because the number of pins available is
tightly constrained. A block diagram of an example implementation
of such a physical backchannel is shown in FIG. 1. A serial
transmission system makes use of a SerDes 100. The SerDes 100 is a
Serializer 106 at the transmitter, and a DeSerializer 108 at the
receiver. The Serializer 106 and Deserializer 108 form a portion of
each of a first application-specific integrated circuit (ASIC) 102
and a second ASIC 104 respectively. The SerDes 100 can be
implemented a transmitter macro 106 and a receiver macro 108. Those
skilled in the art will appreciate that in an ASIC, the term macro
may refer to a somewhat predefined physical design providing a set
of functions, the macro design can be used in the implementation of
any of the number of different ASIC designs. A data channel 110
allows the transmitter 106 to transmit data to the receiver 108.
Physical backchannels 112 are created using physical hardware, such
as pins and wires of a data connection. These backchannels 112 may
be unidirectional or bidirectional, depending on the physical
hardware set aside for them.
[0006] The data link may also be used to communicate performance
information or other metadata, but not during operation. Existing
standards and implementations use existing channels to pass data
between chips at startup time as part of a hand-shaking procedure.
This hand-shaking usually consists of two parts: auto negotiation
(AN) and link training (LT). Auto negotiation is used mainly to
configure both sides (TX and RX) to use the same standard, duplex
mode and data rate. Link training is used mainly to configure TX
amplitudes and equalizer settings. This communication typically
happens at lower speed and requires that high speed pseudo-random
bit sequence (PRBS) be transmitted periodically so that clock and
data recovery (CDR) remains phase-locked.
[0007] Because this all occurs only at start up, it cannot respond
to any changes in conditions during operation. Link parameters must
therefore be set pessimistically, which hurts efficiency. This also
affects the speed at which links can be turned on, which further
hurts efficiency.
[0008] Some techniques have been developed to embed analog
back-channel communication on existing data lines.
[0009] For example, changing the common mode level of the
differential TX or RX circuits may allow some metadata to be
embedded in the data signal while the link is operational. One such
technique is disclosed by A. Ho, et al., "Common-Mode Backchannel
Signaling System for Differential High-Speed Links", IEEE Symp.
VLSI Circuits, June 2004.
[0010] However, it is difficult for this kind of modulation
technique to have no impact on the signal integrity of the data,
especially at very high data rates. It also requires a differential
(two-wire) electrical physical link--it is not suitable for optical
links or single-ended (one-wire) electrical links.
[0011] Another similar modulation technique is disclosed by P. Ta,
et al., "Using Frequency Divisional Multiplexing for a high-Speed
Serializer/Deserializer with Back Channel Communication", U.S.
Patent Application Number 20110038386, Published Feb. 17, 2011.
SerDes links are sometimes alternating current (AC) coupled--where
they are, the low-frequency part of the spectrum may be used for
back-channel communication.
[0012] FIG. 2 shows an example implementation of such a technique
from that publication. A circuit 300 is shown having a first SerDes
302 and second SerDes 304. The first SerDes 302 has a forward
channel driver 306 as well as a receiver 308 for reverse channel
communication; the second SerDes 304 has a reverse channel driver
310 and a receiver 312 for forward channel communication. Two AC
coupling capacitors 314,315 enable the circuit to utilize frequency
division multiplexing, which enables bi-directional transmission
across a communications medium 316. The forward channel passes
relatively high frequency signals output by the forward channel
driver 306 through a first AC coupling capacitor 314, which are
transmitted through the communications medium 316 and passed
through a second AC coupling capacitor 315 to be received by the
forward channel receiver 312. On the reverse channel 318, the
reverse channel driver 310 passes relatively low frequency signals,
which bypass the second AC coupling capacitor 315 through DC
coupling, are transmitted through the communications medium 316,
bypass the first AC coupling capacitor 314, and are received by the
reverse channel receiver 308 through DC coupling.
[0013] However, this technique is not applicable to links that are
not AC coupled. It requires additional pins and external capacitors
in order to set the low-frequency cutoff correctly. It also adds
complexity to the analog data path, which may introduce noise or
other non-idealities. As with the common mode modulation technique
disclosed by Ho et al., it requires an electrical physical link and
so is not suitable for optical links.
SUMMARY
[0014] The present disclosure describes example devices and methods
to communicate information between compatible devices, such as a
SerDes transmitter and a SerDes receiver, using signal reflection
in a communication link, without interfering with the payload data
or increasing the number of pins or physical wire connections
required between the devices (such as TX and RX SerDes macros).
This can be used as a side-channel or back-channel to communicate
metadata, information about the channel or signal quality, etc.
Specific embodiments are described that require minimal additional
power consumption, die area, and cost compared with existing common
SerDes architectures.
[0015] According to some aspects, the present disclosure describes
a receiver for receiving a data signal over a communication link
and sending back-channel data over the communication link. The
receiver comprises a first resistive element having an adjustable
first resistance; a second resistive element having a second
resistance; a terminator for differentially terminating the
communication link using the first resistive element and the second
resistive element; and a back-channel data encoder. The adjustable
first resistance of the first resistive element is adjusted based
on a received first resistor tuning signal. The back-channel data
encoder receives a back-channel data signal and provides a first
resistor tuning signal to the first resistive element based on the
back-channel data signal.
[0016] According to a further aspect, the disclosure describes a
transmitter for transmitting a data signal over a communication
link and detecting back-channel data sent over the communication
link. The transmitter comprises a terminator for differentially
terminating the communication link; a detector for detecting signal
reflections in the communication link; and a back-channel data
decoder for decoding back-channel data from the signal reflections
detected by the detector.
[0017] According to a further aspect, the disclosure describes a
receiver for receiving a signal from a transmitter over a pair of
transmission lines and for transmitting back channel information
towards a transmitter. The receiver has a back channel encoding
controller for generating a control signal in accordance with data
intended for transmission to the transmitter. The receiver also has
a differential terminator connected to the pair of transmission
lines for providing a termination to each of the transmission
lines, the termination of each of the transmission lines being
differentially applied in accordance with the control signal
generated by the back channel encoding controller.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Reference will now be made, by way of example, to the
accompanying drawings which show example embodiments of the present
application, and in which:
[0019] FIG. 1 is a block diagram showing a known example
implementation of backchannels created between the transmitter and
receiver portions of a SerDes using dedicated physical
hardware.
[0020] FIG. 2 is a known example implementation of a system for
creating a backchannel between an AC-coupled SerDes transmitter and
receiver portions by using the low-frequency part of the spectrum
for back-channel communication, as described in "Using Frequency
Divisional Multiplexing for a high-Speed Serializer/Deserializer
with Back Channel Communication", U.S. Patent Application Number
20110038286, Published Feb. 17, 2011.
[0021] FIG. 3 is a block diagram of a first example embodiment
showing a receiver configured to use tunable differential
termination of a communication link to communicate back-channel
data to a transmitter over the communication link.
[0022] FIG. 4 is a voltage amplitude plot of a first data signal
and a second data signal sent over a first conductor according to a
simulation of an example embodiment.
[0023] FIG. 5 is a voltage amplitude plot of a first data signal
and a second data signal sent over a second conductor according to
a simulation of an example embodiment.
[0024] FIG. 6 is a block diagram of an example communication system
using differential termination of communication lines to create a
back channel between a transmitter and a receiver.
[0025] Similar reference numerals may have been used in different
figures to denote similar components.
DESCRIPTION OF EXAMPLE EMBODIMENTS
[0026] The present disclosure describes example devices and methods
that enable a device to create signal reflections in a
communication link to create a backchannel. By varying the
termination impedance or resistance of the communication link, the
device creates signal reflections in the communication link that
can be detected and decoded by the device on the other end of the
link. The termination impedance is modulated to encode backchannel
data the sending device intends to communicate via the backchannel.
The sending device may be either a receiver or a transmitter with
respect to the main data channel flowing over the communication
link.
[0027] In many implementations, a single physical link will be
dedicated to transmitting data in a single direction. This
simplifies communications as a second dedicated link can be used
for transmitting data in the opposite direction without increasing
the signalling overhead associated with the bi-directional
communications. This may be particularly useful in situations
involving transmission of real time data over relatively short
connections
[0028] One issue that may arise is the need for a control channel
over which the receiver can transmit information, such as control
channel data, to the transmitter. As discussed above, this
so-called back-channel information has conventionally been
transmitted either in a separate channel (increasing the complexity
of implementing transmitters and receivers) or through allocating
some of the physical channel resource to the back channel (which
reduces the capacity of the channel).
[0029] In the embodiments discussed below, a mechanism for
transmitting data from a receiver towards the transmitter will be
disclosed. In a receiver, a differential signal can be received
over a pair of wires. These physical channels are typically
logically paired, and the received signal is decoded as a function
of a comparison of the voltage received on each wire. For such a
system to work, each of the wires has to be terminated at the
receiver, which is typically achieved through the use of a
resistive (or inductive in some embodiments) element between the
wire and electrical ground. The exact resistance used is often a
function of characteristics of the operating environment. To
accommodate such variation, each wire is often terminated with a
variable resistance that can be adjusted to select the desired
termination resistance.
[0030] Improper selection of the resistance typically creates an
undesirable voltage across the transmission wire that reduces the
effective bandwidth of the transmission channel. This voltage is
both detectable at the transmitter (especially in comparison with
the paired wire) and controllable through the variable terminator
resistance.
[0031] These, typically undesirable, effects of varying the
terminating resistance are, as noted above, detectable at the
transmitter. Embodiments of the present invention take advantage of
the detectability, at the transmitter, of variation in the
termination at the receiver to create a back channel. By varying
the termination characteristics of the transmission lines, a
voltage change can be detected at the transmitter. This effect may
be more detectable when the termination characteristics of the
transmission lines are varied with respect to each other, which may
include varying the termination characteristics of one of the two
lines. Those skilled in the art will appreciate that in some
embodiments, the termination of a line is provided by at least one
of a resistance or an inductance. In such cases, the termination
characteristic being varied may be the terminating resistance or
the terminating inductance.
[0032] It should be understood that varying the termination
characteristics greatly or quickly may have adverse effects on the
bandwidth of the transmission line, or the quality of the received
signal. However, it is possible to control the magnitude of the
change to a relatively small variance, which results in a voltage
differential across the transmission lines that can be detected at
the transmitter. These relatively small variances will have a
limited effect on the quality of the transmission lines, and this
only needs to happen when there is information to be provided to
the transmitter. This allows any effect of the bandwidth or channel
quality reduction to be limited to the times at which the
backchannel is used for transmission. Furthermore, in many
implementations the change in the properties of the transmission
channel may be detectable, but still within an a priori defined
range of acceptability.
[0033] Phrased another way, the signal reflections may allow a
receiver (Rx) to talk to a transmitter (Tx) through the same
physical Tx-to-Rx channel. This would avoid requiring a dedicated
back-channel or a separate return (Rx-to-Tx) channel for the RX
chip to send data to the TX chip. It would also enable each
physical channel in a multi-channel communication system to have
its own back-channel. Because each back-channel would be paired to
its own dedicated channel, they could operate independently and in
parallel with each other.
[0034] The back-channel data can be sent from the Rx to the Tx by
encoding it in a signal that is generated by skewing the
differential termination impedance of the communication link, i.e.
by skewing the termination applied to the RxP wire (by a "plus" or
"positive" polarity termination resistor) and the RxM wire (by a
"minus" polarity termination resistor, also referred to as RxN or
"negative" polarity). This would in turn result in a difference in
the corresponding signal amplitudes of the TxP wire and TxM wire
measured at the Tx side of the communication channel. A low
frequency amplitude or peak detector could be used to detect this
change in TxP and TxM amplitude.
[0035] With reference to FIG. 5, a communication system 500 is
shown encompassing a transmitter 510 and a receiver 530 in
communication via a differential communication link 550. The
communication link 550 comprises a first conductor 552 and a second
conductor 554 for carrying a differential data signal; in this
illustrated embodiment, the first conductor 552 is framed as a
positive lead (txp to rxp), and the second conductor 554 is framed
as a positive lead (txm to rxm or txn to rxn), but this polarity
could be reversed without affecting functionality.
[0036] The receiver 530 includes a terminator 532 for
differentially terminating the communication using a first
resistive element 534 having an adjustable first resistance R_rxp
536 and a second resistive element 538 having an adjustable second
resistance R_rxm 540. Those skilled in the art will appreciate that
an adjustable resistance may also be referred to as a variable
resistance. The first resistance R_rxp 536 is adjusted by a
received first resistor tuning signal 592, and the second
resistance R_rxm 540 is adjusted by a received second resistor
tuning signal 594. The signal carried by the first conductor 552 is
received at the receiver, and is terminated by a connection to a
ground 542 via the first resistive element 534, and to a first
receiver circuit 572. The signal from the second conductor 554 is
connected to the ground 542 via the second resistive element 534,
and to a second receiver circuit 574.
[0037] The receiver 530 transmits back-channel data 590 to the
transmitter 510 by encoding the back-channel data 590 as signal
reflections in the communication link 550. The receiver 530
includes a back-channel data encoder 580 that receives back-channel
data 590 and controls a differential termination of the paired
communication links. This differential termination can be effected
by generating at least one of the first resistor tuning signal 592
and the second resistor tuning signal 594. By adjusting or tuning
the first resistance R_rxp 536 and the second resistance R_rxm 540,
the impedance of the terminator 532 is adjusted over time based on
the back-channel data 590. The back-channel data encoder 580 thus
encodes the back-channel data 590 as signal reflections in the
communication link 550, as the change in termination impedance over
time creates varying signal reflections over time.
[0038] In the illustrated example, the transmitter 510 also has a
transmitter terminator 512 comprising a transmitter first resistive
element 514 having an adjustable transmitter first resistance T_rxp
516 and a transmitter second resistive element 518 having an
adjustable second resistance T_rxm 520. The transmitter terminator
512 differentially terminates the communication link 550 on the
transmitter side. This may allow the TX to use the same
reflection-based back-channel communication technique to
communicate with the Rx during link operation, enabling
bidirectional back-channel communication. However, a system may
also implement uni-directional back-channel communication from the
RX to the TX side without the use of adjustable resistive elements
on the TX side. In a uni-directional application, resistive
elements 514 and 518 may have fixed resistance instead of
adjustable resistance.
[0039] The transmitter decodes the back-channel data 592 from the
communication link 550 by means of an amplitude detector 522 in
communication with the first conductor 552 and second conductor
554. The amplitude detector 522 can detect the changes in
receiver-side termination impedance over time due to the changes in
signal reflection present in the communication link 550.
[0040] Some embodiments implement each of the resistive elements
514,518,534,538 as a plurality of resistors connected in parallel
or termination slices. Switches may be used to activate or
deactivate each of these parallel resistors or slices to increase
or decrease the overall resistance provided by the resistive
element.
[0041] The encoding or modulation scheme for the back-channel data
is, in some embodiments, designed to minimize disruption of the
data signal from the transmitter 510. The signal reflections
created through modulation of termination impedance are small in
amplitude and slow in frequency to avoid significant degradation of
signal-to-noise ratio (SNR) in the data signal. Disruption of data
signal integrity (SI) can also be minimized by only sending
information over the back-channel for a short time during mission
mode (i.e. when the data signal is being actively transmitted by
the transmitter 510 over the communication link 550).
[0042] By using low-frequency, low-amplitude changes in termination
impedance at the receiver 530, noise can be minimized on the
communication link. No common-mode noise is introduced over the
communication link, and the back-channel does not introduce any
differential offset, so the signal slicing point at the receiver
530 is not affected.
[0043] A first example encoding scheme uses the relative resistance
values of R_rxp 536 and R_rxm 540 to encode bits of back-channel
data 590. When R_rxp 536 is set to be greater than R_rxm 540 during
a predetermined back-channel time unit interval (thereby creating a
detectable decrease in TXP amplitude relative to RXP amplitude at
the transmitter 510), this encodes a "1" bit value. When R_rxp 536
is set to be less than R_rxm 540 during a predetermined
back-channel time unit interval, this encodes a "0" bit value
(thereby creating a detectable increase in TXP amplitude relative
to RXP amplitude at the transmitter 510). When R_rxp 536 is set to
be equal to R_rxm 540 during a predetermined back-channel time unit
interval, this encodes no data.
[0044] The adjustment of R_rxp 536 and R_rxm 540 may be
accomplished by adjusting one value, or both. The value or values
may be adjusted upward or downward, by the same amount or by
different amounts. For example, an embodiment may maintain both
R_rxp 536 and R_rxm 540 at fifty ohms (50.OMEGA.) as a baseline
resistance encoding no data. To encode a "1" bit value in the
back-channel, R_rxp 536 may be adjusted to fifty-five ohms
(55.OMEGA.) and R_rxm 540 may be adjusted to forty-five ohms
(45.OMEGA.). A "0" bit value may be encoded by adjusting R_rxp 536
to forty-five ohms (45.OMEGA.) and R_rxm 540 to fifty-five ohms
(55.OMEGA.).
[0045] In some embodiments, the transmitter 510 terminates the
communication link 550 using resistors having adjustable
resistance, such as transmitter first resistive element 514 having
an adjustable transmitter first resistance R_txp 516 and a
transmitter second resistive element 518 having an adjustable
second resistance R_txm 520. In such embodiments, the
transmitter-side resistive elements 514,518 may adjust their
respective resistances 516,520 in inversely proportional response
to a detected change in the receiver-side resistances. Thus, in one
such example embodiment, when the transmitter-side amplitude
detector 522 detects an increase in amplitude TXP of the signal
being transmitted over the first conductor 552, corresponding to a
decrease in the value of resistance R_rxp 536, the transmitter 510
responds by effecting a corresponding increase in resistance R_txp
520. This negative feedback system would maintain a constant
amplitude of each of the two signals being sent over the conductors
552,554, thereby minimizing any effect of back-channel reflections
on data signal integrity.
[0046] In some examples, the impedance changes are small relative
to a baseline resistance or impedance level. For example, a
baseline impedance of the receiver-side terminator may be 50.OMEGA.
in one embodiment. If R_rxp 536 is skewed to 60.OMEGA. and R_rxm to
40.OMEGA., the transmitter differential amplitude may be kept
constant at about .about.1V. The peak to peak amplitude of TXP
would be .about.100 mV larger than TXM. However, in example
embodiments, such a large impedance change may not be necessary. A
change in R_rxp and R_rxm of 5.OMEGA. each may be sufficient in a
system with a 50.OMEGA. baseline.
[0047] The amplitude detector 522 may in example embodiments be an
amplitude or peak detector to compare the difference in
single-ended amplitudes between TXP and TXM at the TX side. Because
the data is low frequency in some examples, this detector can be
small and low power.
[0048] Creation of a bi-directional backchannel is possible in some
embodiments. Where the transmitter-side impedance (R_txp 516 and
R_txm 520) is adjustable, low frequency data can be transmitted
from the transmitter 510 to the receiver 530 by skewing the
transmitter drive resistances R_txp 516 and R_txm 520. This
bi-directional back-channel communication can in some examples
enable hand-shaking or auto negotiation for each communication
link. In some examples, this may enable a single communication link
to coordinate bi-directional link training, adaptation, or both
without relying on any other communication link between the
receiver 530 and the transmitter 510.
[0049] In some embodiments with such bi-directional back-channel
communication, the transmitter 510 and receiver 530 would establish
separate time windows for receiver-side back-channel use and
transmitter-side back-channel use to avoid back-channel data
collision. In other embodiments, some other form of sequential
turn-taking is established between the receiver 530 and transmitter
510 for the same purpose.
[0050] Some devices or systems already use receivers and/or
transmitters with adjustable termination impedance, including
termination impedance using multiple parallel termination slices.
Implementing the described examples in the context of such systems
or devices could potentially be very simple to implement, and would
potentially have zero impact on termination loading or
bandwidth.
[0051] FIGS. 4 and 5 show simulation results of example SerDes
implementations of the described techniques. FIG. 4 is a voltage
plot of two positive-polarity data signals sent by the transmitter
510 over the first conductor 552: a first output signal amplitude
610 and a second output signal amplitude 620, measured in response
to two different adjustments of the receiver impedance. The first
output signal amplitude 610 is in response to an adjustment of
R_rxp 536 to 45.OMEGA. from a baseline of 50.OMEGA., and an
adjustment of R_rxm 540 to 55.OMEGA. from a baseline of 50.OMEGA..
The second output signal amplitude 620 is in response to an
adjustment of R_rxp 536 to 55.OMEGA. from a baseline of 50.OMEGA.,
and an adjustment of R_rxm 540 to 45.OMEGA. from a baseline of
50.OMEGA..
[0052] Similarly, FIG. 5 shows a voltage plot of two
negative-polarity data signals sent by the transmitter 510 over the
second conductor 554: a first output signal 710 and a second output
signal 720, measured in response to two different adjustments of
the receiver impedance. The first output signal 710 is in response
to an adjustment of R_rxp 536 to 45.OMEGA. from a baseline of
50.OMEGA., and an adjustment of R_rxm 540 to 55.OMEGA. from a
baseline of 50.OMEGA.. The second output signal 720 is in response
to an adjustment of R_rxp 536 to 55.OMEGA. from a baseline of
50.OMEGA., and an adjustment of R_rxm 540 to 45.OMEGA. from a
baseline of 50.OMEGA..
[0053] The first output signals 610,710 can therefore be considered
to represent a data signal being sent during transmission from the
receiver 530 to the transmitter 510 of a first bit value of
back-channel data, such as "1". The second output signals 620,720
can correspondingly be considered to represent a data signal being
sent during transmission from the receiver 530 to the transmitter
510 of a second bit value of back-channel data, such as "0". These
simulation results show that the data signal is minimally perturbed
by the transmission of back-channel data via differential
termination impedance in an example system where in the baseline
impedance is 50.OMEGA. and the resistance 536,540 of each resistor
534,538 is changed by only 5.OMEGA. to encode a bit of back-channel
data 590.
[0054] FIG. 8 is an illustration of a communication system 800 that
uses differential termination of communication (or transmission)
lines to create a back channel between a transmitter 802 and a
receiver 804. Transmitter 802 and receiver 804 are connected by a
communications link 808. In the illustrated embodiment,
communications link 808 has a pair of transmission lines 810 and
812. In one embodiment, the paired transmission lines 810 and 812
are used to carry a differentially encoded signal. Transmitter 802
has a pair of transmitting amplifiers 814 and 816, the outputs of
which are connected to lines 810 and 812 respectively. In some
embodiments the output of the amplifier 814 and amplifier 816 is
connected to transmission line 1 terminator 818 and transmission
line 2 terminator 820. Signals transmitted over transmission line
810 is received at amplifier 822 in receiver 804, while the signal
transmitted over transmission line 812 is received by amplifier
824. As will be understood by those skilled in the art each of the
transmission lines 810 and 812 are terminated at the receiver. In
many conventional implementations, a variable termination is
provided for each of the transmission lines 810 and 812. This
allows each line to be terminated in a manner that allows for a
maximization of the bandwidth of the channel 808. What the prior
art would have considered as an improper termination of
transmission lines 810 and 812 will result in a detectable change
in a voltage detectable at the transmitter 802. Differential
terminator 826 controls the termination of the transmission lines
in accordance with a control signal associated with data intended
for transmission over the back channel provided by back channel
encoding controller 828. Differential terminator 826 can, based on
the received control signal, can set the termination for each of
the transmission lines 810 and 812 to different values. In some
embodiments, an intentional skewing of the termination is applied,
while in other embodiments one of the two transmission line
terminations is modified. This creates a detectable change in
voltage at the transmitter 802, and if a differential termination
is applied, this may be manifested by a detectable difference in
the voltage between the lines 810 and 812. Back channel decoder
830, in transmitter 802, detects changes in the voltage
characteristics on transmission lines 810 and 812. A change in the
amplitude of voltage carried on each of the lines 810 and 812, and
in some embodiments the change in the amplitude of the voltage of
line 810 with respect to the voltage of line 812 can be detected.
This change in the detected voltage carries the back channel
signal, which can be decoded and acted upon by transmitter 802.
[0055] In some embodiments, transmitter 802 includes the back
channel encoder 832 illustrated as optional. This allows the
control of the termination applied by terminators 818 and 820. The
control of termination at the transmitter can allow the transmitter
802 to create a detectable voltage change across lines 810 and 812
that can be received by a back channel decoder (not illustrated) at
receiver 804. This optional embodiment allows for control
information to be encoded in voltage changes by each side of the
channel.
[0056] In some embodiments, the back-channel data comprises link
performance information which is available only at the receiver
side, but can be used by the transmitter side, e.g. for link
training and/or adaptation of a transmitter-side continuous time
linear equalizer (CTLE).
[0057] Some embodiments may adjust the resistance of only one
resistor of a differential pair. For example, a receiver configured
to send back-channel data may encode the back-channel data by
varying the first resistance of the first resistive element 534
without adjusting the resistance of the second resistive element
538. This may sacrifice some of the signal stabilizing effects of
adjusting the resistances in opposing directions but may simplify
implementation.
[0058] Various aspects of the above-described embodiments could be
recombined to form additional example embodiments.
[0059] Some embodiments may require a particular hardware
configuration in TX and/or RX macros of the SerDes, whereas others
may be implemented purely as firmware or other software.
[0060] Various embodiments may be applied to applications operating
according to various short- or long-haul communication standards,
including Optical Internetworking Forum (OIF) standards, IEEE
10GBASE-KR, IEEE 25GBASE-KR, and other known SerDes communication
standards.
[0061] The communication link 550 may in some embodiments be an
electrical link, such as a twisted-pair cable or a backplane system
bus.
[0062] Various embodiments would be usable with various types of
physical electrical communication links.
[0063] In some embodiments, the SerDes may use conventional
auto-negotiation (AN) and link training (LT) techniques at startup,
in order to comply with existing standards, and then make use of
one of the described techniques in the background, for dynamic
adaptation while the data link is operational.
[0064] Some embodiments may apply the described techniques outside
of the context of SerDes. Any receivers and transmitters
communicating across an electrical link can potentially apply the
described techniques for creating a back-channel within the
communication link by modulating termination impedance at the
receiver and/or transmitter side, thereby creating time-varying
signal reflections within the link.
[0065] Some embodiments may be compatible with the use of other
back-channel communication techniques, such as common-mode
modulation, as long as the limitations of such techniques as
described in the Background section are taken into account.
[0066] Although the present disclosure describes methods and
processes with steps in a certain order, one or more steps of the
methods and processes may be omitted or altered as appropriate. One
or more steps may take place in an order other than that in which
they are described, as appropriate.
[0067] The present disclosure may be embodied in other specific
forms without departing from the subject matter of the claims. The
described example embodiments are to be considered in all respects
as being only illustrative and not restrictive. Selected features
from one or more of the above-described embodiments may be combined
to create alternative embodiments not explicitly described,
features suitable for such combinations being understood within the
scope of this disclosure.
[0068] All values and sub-ranges within disclosed ranges are also
disclosed. Also, although the systems, devices and processes
disclosed and shown herein may comprise a specific number of
elements/components, the systems, devices and assemblies could be
modified to include additional or fewer of such
elements/components. For example, although any of the
elements/components disclosed may be referenced as being singular,
the embodiments disclosed herein could be modified to include a
plurality of such elements/components. The subject matter described
herein intends to cover and embrace all suitable changes in
technology.
* * * * *