Posted under PIC because this rather long rant relates directly to my=20
frustrations with the J-series PICs.

I've been prototyping some new hardware, based around a PIC18F85J50 and=20
an FPGA. The FPGA handles the number crunching, the PIC is a=20
general-purpose garden-variety USB to parallel bridge with some smarts=20
(the FPGA allows it to share the ACQ RAM).

The FPGA, however, is not the problem. The PIC is.


1) The pin-one marker on my parts is ambiguous.
There are two round depressions in the chip face; one is a moulding=20
mark, the other is the Pin 1 marker. How do you tell them apart? Hold=20
the chip with the text horizontal and the Microchip logo at the top. Pin=20
1 is at the bottom left. Also, the moulding mark has a bit of text=20
inside it, though you need a strong magnifier and a bit of chalk (or a=20
paint marker and some IPA) to see it.

This cost me one chip. =A34 down the pan. Thanks, Microchip.


2) The MCLR pin can't handle 12V.
MCLR on these chips is specced to run at about 5V *absolute maximum*.=20
The real limit is Vdd (usually 3.3V). If you change the device type=20
during an MPLAB run, MPLAB may not save that device type if it crashes...

I just had an MPLAB crash. Restarted, and selected the PICKit2. MPLAB=20
had reselected the PIC18F252 I was using before. BANG goes another chip.=20
=A38 down the pan.

I think I'm going to solder a 4.1V Zener diode across the MCLR test pad=20
and a conveniently located ground via...

Tomorrow I get to fire up the SMD workstation and replace this chip.=20
Which is going to be great fun since it's surrounded by 0603 decoupling=20
capacitors and my vacuum pick-up pen is shot. Blu-tak on a cocktail=20
stick, anyone?

And I haven't even gotten to a point where I could upload the LED=20
blinker or FPGA boot test code yet.

AARGH. Sometimes it's just not worth chewing through the leather straps=20
in the morning... I'm going to bed.

TTFN,
--=20
Phil.
piclist@philpem.me.uk
http://www.philpem.me.uk/

--=20
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