Folea, UniversalScan - Ricreations,
full version of this article including all the graphics can be viewed here
with graphics, pictures and screen shots]
boundary scan tools are very effective in manufacturing test for
diagnosing board problems - especially when the board is designed to
optimize the use of these tools. In concept it is very simple - you
hand the tools your net list, they automatically calculate all the
necessary test vectors, apply the vectors, read/interpret the results
and tell you where the discrepancies are on your board.
practice, though, setting this up can be a very tedious and time
consuming exercise - especially if the board is a prototype that has
no test fixtures, an unstable net list and questionable CAD data after
numerous undocumented cuts and jumpers.
debugging prototypes you are usually trying to figure out what is
going on in a very small area of the board and don't really want to
take the time or trouble to setup all the test vectors, etc. - you
just want to see if a few pins have continuity and you want the answer
new genre of boundary scan tools is available now that remove the
tedium of traditional boundary scan test. Within seconds, you can
manually see and control every scan enabled pin under every JTAG
device on your board in real time without having to create test
vectors, test executives, test projects, etc. You just drop the
parts on the screen, connect the chain to your PC and hit SCAN.
Review from a New Perspective
The idea behind
boundary scan is simple enough – connect a latch to every I/O and
buffer enable on a device so you can control and monitor the I/Os
independently from the internal logic.
Connect those latches in a long shift register so you can shift
the results in/out. The
result looks like Figure
yellow boxes represent the latching logic – scan cells - that are
used to capture and monitor the I/O in and out of the device.
these are transparent and the user and the device don’t even know
they are there.
this mode, called SAMPLE/PRELOAD, you instruct the yellow boxes to
unobtrusively take a snapshot of the signals and then shift the
results out TDO to view them.
sampling and shifting operation is completely independent of the
operation of the device and internal logic.
You can do this all day long without affecting the device’s
operation in any way. If you do this over and over and display the
result in the same form factor as your device, then you can see a real-time
display of pin activity.
beauty of this is you can now instantly see if a signal is high, low
or toggling – even if the signal is buried under a BGA package and
while your circuit is running. For
example, here is a JTAG chain of a configuration PROM and an FPGA:
2: Viewing Pin activity in real time on your PC Display.
this screen shot you see the parallel port is connected via JTAG to
the configuration PROM, which is connected to the FPGA which is
connected back to the parallel port.
The red, black and blue dots represent the pins on the parts.
Red indicates a pin is at a logic high, black indicates it is
at a logic low and blue indicates pins you can’t scan with boundary
scan (power, ground, the JTAG signals themselves, etc).
a pin is toggling between red and black, then it is actively being
driven. All of the sudden it has become quick and easy to see the
status of a signal. You just look at the pin – if it is blinking
then there is activity on that pin, if it is red the signal is high
and if it is black the signal is low.
you measure a 12MHz oscillator with boundary scan? No, boundary scan isn’t fast enough for that. Remember, you
have to tell the scan cells to capture the values, then shift them all
out, then capture again, then shift out again, etc.
This takes way too long to do any high speed measurements. But,
let’s face it, most of the time when you connect an O-Scope to a
signal you just want to know if the signal is high, low or toggling
– you are looking for an indication of activity – and that is
exactly what these tools provide, except they can do it in places
where the O-Scope can’t get to!
best news is boundary scan isn’t just passive.
In addition to the SAMPLE/PRELOAD mode we discussed above,
there is also an EXTEST mode where you tell the scan cells (the yellow
boxes in Figure 1.) to disconnect the signals from the internal logic
and allow the scan cells to control the I/Os directly.
EXTEST you shift data into the scan cells via TDI, then tell the scan
cells to apply the data to the I/Os. You now have total control over
every JTAG enabled I/O on the device. You can enable buffers, drive
buffers, and monitor signals simply by putting 1’s and 0’s into
the appropriate scan cells.
do you control the values that get put into the scan cells?
Simple – connect a virtual switch. We’ll use this concept
in the next several sections to show you a new way to test and debug
Boundary Scan for Low Level Debug
Simple continuity testing
example in Figure 3 shows a Motorola processor and an ASIC in the JTAG
3: A Motorola processor and an ASIC.
Virtual Switches and LEDs are used to monitor and control the
address bus pins.
this example we have connected virtual 7-segment LEDs to the Data and
Address buses so we can see what they are doing at a glance. On the
right side there are 4 columns of devices: the first column are toggle
switches and are connected to the output buffer enables on the address
bus of the processor. They are lit, indicating a logic 1 is being
applied to the enable. The
second column of devices are momentary switches that are connected to
the input of the output buffer. The
third column of devices are virtual LEDs connected to the input
buffers of the processor, and the fourth column of devices are virtual
LEDs connected to the input buffers on the ASIC.
do a simple continuity test, we just set the buffer enables, drive the
output buffers with the momentary switches and watch the LEDs.
If both LEDs light up, then the input buffers on both devices
saw the logic level change and we know we have continuity.
figure 3 you can see where we pressed the momentary switch on the 2nd
row to drive an address line and both LEDs lit up.
We have continuity. When
we pressed the switch on the 4th row, only the Processor
LED lit up – the signal didn’t make it to the ASIC.
all there is to it – just click on switches to put values in the
scan cells, and watch the results on LEDs.
Quick and easy.
example: The yellow LED on the ASIC is connected to the 80MHz
Oscillator. We see the
Virtual LED toggling so we know the oscillator is connected to the
ASIC. If we saw it stuck
at a high or a low, then we would know we have a problem.
of the problems with most boards is testing IO’s without a test
fixture. Most of the time you just want to see if a pin under a BGA is
physically connected to an I/O on your board.
With these tools it is easy – just toggle the pin manually
and monitor the pin on the connector with an O-Scope or DVM.
you have mapped out your test circuit, you can rearrange the virtual
switches and LEDs on the screen to create a quick and dirty control
panel from your board. You can even use this to drive/control
circuitry before firmware is written.
Activate a solenoid, turn on a light, drive a D/A, etc.
boundary scan gives you total control over the pins on a device, and
if you have a Flash device connected to those pins, why not use
boundary scan to program the Flash device?
This is a great way to get access to a Flash memory without
having to provide extra circuitry or means to remove it from the
circuit. You just tell the tool which pins in the chain are connected
to the Flash Device, specify a data file and hit PROGRAM.
The tool automatically configures the device, erases it,
programs it and verifies it.
is a great way to independently program your Flash memory in system
without a functioning processor to do it.
This is a great way to get a boot loader into a Flash memory
for a processor, then let the processor come up and load the rest of
brings up the downside of Flash programming with Boundary Scan – it
can be VERY slow. A 10K-Byte file can take a few seconds to several
minutes (depending on the tools, options and length of your scan
the problem – we have to shift the entire scan chain just to change
the state of a single pin. That
means to write a single byte we have to:
Shift the entire chain to setup address and data
Shift the entire chain to enable the write-enable
Shift the entire chain to disable the write-enable.
your typical scan chain is a couple thousand bits (usually 3 scan
cells per pin, times the number of pins.) then you will have to do 5
to 10 thousand shifts just to write one byte.
Repeat that for every byte in your file and you can see why it
will take some time.
good news is new faster USB based pods are coming out that will
drastically reduce this programming time.
The traditional boundary scan tools also offer several high
speed programming options and plug-in modules.
story as above – simply tell the tools how the SPI device is
connected to the JTAG chain, specify a data file and hit PROGRAM.
Quick, easy, simple.
all those little virtual switches all the time gets real tedious real
fast. Fortunately, you
can record a test session on a known good board, then swap boards and
hit PLAY to repeat the exact same test.
Using this you can quickly build a little prototype test
fixture for your early boards without having to worry about
test-vectors, CAD data, ...
is important to understand that these tools do not replace the
traditional boundary scan tools - they augment them. These low level
manual debug tools are great for prototype debug in the lab and for
learning about boundary scan (and convincing management to invest in
the high end traditional tools!) because they are quick, simple, easy
and inexpensive. The high-end automated tools are great for diagnosing
high volume mature boards in production – especially if those boards
are properly designed for automated test!
key difference is that these new low-level tools provide real-time
display and control of a board while it is running.
The traditional tools typically do full board scans on boards
that are not running.
More Information …
examples used in this article are from Ricreations “Universal
and Macraigor Systems “J-Scan” (www.j-scan.com).
Check out those websites for more examples, free trials and tutorials.