March
1999, Issue 104
JTAG—Working
with CoolPLD
by
Jeff Bachiochi
A
lot of manufacturers are seeing the advantages of
JTAG, especially when it comes to in-circuit serial
programming. Join Jeff as he takes a look at one of
the low-cost tools that make it all possible.
I
avoided programmable logic (PALs and PLAs) until International
CMOS Technologies (ICT) introduced programmable electrically
erasable logic (PEEL) in 1987. Before this, there were
only OTP devices that gobbled up high currents compared
to standard logic devices.
Companies
were rather hush-hush about their proprietary programming
algorithms. Special programming voltages were necessary,
which required special programmers.
If
you happened to be at a company with established programming
hardware, you might be able to get a hold of the algorithms.
But, as for you or me building a programmer, forget
it.
The
PEEL devices added a new twist to the equation—CMOS
devices that were reprogrammable. What did that mean
to the designer using programmable logic devices in
a design? Reprogrammability gave the designer a level
of flexibility that was never achievable before.
Suddenly,
all the logic between the input and output pins of the
device could be altered without having to design a new
PCB. That is, assuming the device was a DIP part and
was inserted into a DIP socket on the PCB.
I
was already convinced that the payback for using IC
sockets on PCBs was justifiable, so using PEELs merged
easily with my mindset. Now, I’m not trying to
tell you that a programmable device can always fix the
problems without PCB redesign, but in many instances,
it does just that.
One
of the simplest examples is using the programmable logic
device as an address decoder. Perhaps you need to move
or swap some I/O selects. A quick change to the logic
equations can reconfigure any or all of the I/O without
any change to the PCB. This procedure also saves cuts
and jumpers to existing boards.
