Download Lior - Test Receiver

THE 1149.6
TEST RECIVER
Presentation In The Framework Of JTAG
Course
Lior Vaknin - June 2005
Introduction
Alternative connection
methodologies for propagating
signals between modules.
The distances signal travels may be
short, for example, a few centimeters
on boards but could range up to
several meters in systems.
A signal is defined as having a source
or transmitter and a
destination, or receiver and travels
along a path, typically a wire.
Examples for Designing Path Factors
The speed of signal propagation is the speed
of light:
=>data rates become high, propagation
speed interact with the distance the
signal must travel.
Signal can be damaged, encoded information
is degraded or lost:
=>unwanted noise is added into the
information signal.
If noise is added, the receiver will sense this
noise as well. However, if the receiver is
designed to use threshold then this noise
voltage will not affect the transmission.
A signal contains power which is to say it
takes energy to transmit information. This
power is a necessary feature, but can lead to
problems, namely heat.
Power can be limited by slowing down the
rate of information transfer, or by limiting the
voltage swing of the signal.
However this is fraught with risk, since as
voltage swings become smaller, the noise
immunity offered by signal thresholding is
lowered as well.
Signal can reflect off the end of the wire and
bounce backward. and adds into the
subsequent data waveforms, which is just like
adding in noise. This phenomenon can be
canceled out by impedance matching
One approach to reducing sensitivity to noise
is to have two thresholds.
However, there is some time uncertainty
about when the receiver will perceive the
transition.
AC Coupling
AC coupling is usually performed by adding a
capacitor to the path.
This capacitor is called a DC blocking
capacitor.
The combination of a series capacitor and
resistor forms a high-pass filter.
This was steady-state response.
But, if the waveform was recently ‘turned on’,
there is a time where the voltage is not
centered at 0, but effectively centered at 2
volts. This is the transient response.
Designers try to avoid this problem by
sending extra bits in the stream that
compensate for imbalances in the data. These
bits are located in known positions and are
later ignored (preload).
Differential Signaling
Use two wires to transmit the same data
stream. One wire is the positive and the other
is the negative.
The receiver compares the two signals,
if Vpos > Vneg producing 1 else producing 0.
How does differential signaling helps ?
First, With mutual references, the
forbidden zone is greatly reduced. (much
more assurance when transition will be
perceived).
Second, we can reduce the actual voltage
swing, the signal can change faster,
better slew rate.
Transition has less ‘distance’ to travel in
the same period of time.
Thus we can achieve higher data rates
with the same edge.
This reduces power and increases the
number of edges we can pack into a time
period – and increases the data rate.
Better noise immunity.
Say the positive leg has a value of P and the
negative leg has a value of N. The subtraction
result is P – N.
Now let’s add a noise error E. This gives us
P+E and N+E (most noise are common mode
noise).
Now the subtraction result is
(P+E) – (N+E) = P – N, as before.
Differential signaling does have costs.
As design requirements for data streams
with gigabit capacities become
commonplace, we find this also
complicates the design. As a
result, each IC vendor confronts these
challenges, they may come up with
somewhat different solutions. This can
lead to ‘single-source’ problems.
AC coupling, as shown here, can help with
these problems.
(Incompatible when DC coupled)
This configuration is also ‘hyperactive’.
If the driver stops transmitting data
The capacitors discharge, Vref appears on
both inputs and the subtracts result is (Vref –
Vref).
The value of 0 is in the uncertainty
region, so the receiver will do indeterminate
things as a result.
To avoid this, the receiver always need some
sort of signal (even with ‘idle’)
The gain with AC coupled differential paths :
-lower signal voltages.
-greatly reducing power consumption
and heat.
-higher operating frequencies.
-higher noise immunity.
This satisfies several needs for high-speed
system design.
How does all this become attached
with JTAG ?
What is EXTEST ?
Who am I ?
THE TEST RECIEVER
Building Blocks
Voltage Comparator
Time Hysteresis
Memory
Reference System - Switchable
DC/AC-Coupled Transition
DC Response
When EXTEST instruction is in
effect, the receiver is expected to
respond to the voltage levels seen at
the input.
AC Response
When the EXTEST_PLUSE or
EXTEST_TRAIN instruction are in
effect, the test receiver is expected to
response to valid signal transitions seen
at the input.
An Integrated AC/DC Receiver
Init Clk Generator
Test Receiver Memory
Initialization
THANK YOU !