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'[PIC] Serial Test Tool, was [PIC] RAM display'
2006\04\03@030335 by Bob Axtell

face picon face
Neat idea.

I developed a manufacturing test tool several years ago, during the
era. This one sends out on ONE pin. It pumps out a manchester code (variable
time between bits, but the information is always a fixed percentage of
the bit width).
I have it send out a  line of 16  bytes, usually (but not always)
advanced one line of
16 with each pass plus 9 bits of RAM ADDRESS info, plus two start bits
and two
stop bits. The total bit count is 16*8  + 9 + 2 + 2 = 141 bits. With a
bit period of 5mS,
it takes 141*5 or 705mS per pass. I post a gap to allow the receiver to

The data looks like this:

_---_-__-          etc (A ONE then a ZERO)

The bit begins with a ONE, and ends when the NEXT bit starts. A ONE is
when the bit pulse with is MORE than 50% of its bit length, a ZERO is
defined as
LESS than 50%. I run it flawlessly at 5mS per bit, but it operates well
at 1mS per
bit as well, You can see that the LAST bit in the bit stream contains NO

The receiver for this is an old DOS 66Mhz laptop. The signal goes into a
pin on a printer port. Together with a ground, that's all there is. A
twisted test
cable can be as far away as 10 meters and recover the data reliably.

The way the data is recovered with two timers. The first one measures time
between bits, and the second measures the time the bit pulse is HIGH. On the
DOS machine, I used a bad serial port (logic worked, RS232 driver was
blown) to provide an internal timer, by sending data as fast as possible
at 115kb
and counting each transmission time. That timer was used as a reliable fast
measurement. The standard tick clock is too slow (18 ticks/sec).

Jinx wrote:

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2006\04\03@043230 by Jinx

face picon face

> Neat idea.

Thanks. PICs can be put to all sorts of uses to test other PICs. A
long time ago I made a logic analyser with an F84. Although it was
only 4MHz, what I did was use a 50MHz crystal driving fast cache
RAM and have the F84 merely supervise and do the upload to a
PC. Got me out of a lot of hardware scrapes and solved more than
a few headscratchers. Since then I've made the odd tool and they
all give a valuable insight as to what's going on

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