Joachim,

> Can someone point me towards a simple test circuit (with code) so that
> I can test my programmmer.  I know there a probably tons out there but I
> can't find one.

I write the following on the PIC list about a year ago. I think it is the
 easiest
possible way (implied challenge, folks ;-)...
The test circuit is trivial: hook up power and crystal, and use a tester
or LED to check frequency.

 -- Martin

Martin Nilsson                           http://www.sics.se/~mn/
Swedish Institute of Computer Science    E-mail: mn@sics.se
Box 1263, SE-164 29 Kista                Fax: +46-8-751-7230
Sweden                                   Tel: +46-8-752-1574

> Subject: Supersimple non-destructive testing of OTP PICs (and others)
> Date: Thu, 06 Jun 1996 16:16:02 +0200
> From: Martin Nilsson <mn@sics.se>
>
> I was thinking about PIC testing, and came across the following
> nice trick. When you know it, it is obvious, but I haven't seen it
> elsewhere, and I think it deserves attention. The trick works on
> all 16C6x-8x, OTPs as well as reprogrammables.
>
> This trick is useful in several ways:
> - As a simple test program that you put into your stock of "fresh" PICs.
>   When you need one you can quickly probe it to see if it has been used,
>   without having to read it in a programmer.
> - The trick works
>   for (partial) testing of OTP devices. It doesn't require changing
>   the configuration fuses, and only destroys one word, which could be
>   located anywhere in memory. The test discovers if execution logic
>   works and may discover if part of the program memory is damaged.
>   It executes all program memory locations.
> - As a test for an in-system programming circuit.
>
> * Ok, the trick is simple:
>
>    TRIS PORTB
>
> anywhere in memory! Leave the rest at factory default settings.
> * GOOD DEVICE: RB7 will switch between HiZ and LowZ with a frequency of
> about F[in Hz] = f[in MHz]/m[in k word], where f is the RC oscillator
> frequency, and m is the program memory size.
> * NG DEVICE: RB7 switches much faster (at least twice F) or much slower.
>
> In other words, the maximally simple blinker program!
>
> Explanation:
>
> The test is the shortest possible PIC program the execution of which
> is externally detectable. PIC program memory defaults to all ones,
> which on the 6x-8x means ADDLW 0xFF, equivalent to W:=W-1.
>
> There is only one zero-instruction program (all ADDLW 0xFF), but the
> execution of this program cannot be observed. The chip could be broken
> but you wouldn't know. You can only see the oscillator oscillates.
>
> There are two one-instruction programs that are observable:
> 1. SLEEP
> 2. TRIS PORTx
>
> All of these will work, but TRIS PORTB is probably preferrable. The
> factory setting for the PIC is Watchdog timer on, with 1:128 prescaler.
> SLEEP means the PIC will run through the memory (ADDLW 0xFF), and then
> SLEEP for about 2.5 s. This will be detectable by a short blip
> on the CLKOUT pin. (The factory setting of the oscillator is RC.)
>
> TRIS PORTA and PORTB will work the same way, but PORTB is more useful.
> On startup, PORTB will contain a random value. W will also contain a
> random value. On every cycle through memory, W will be incremented by
> one (decremented by one n*2^8+255 times), and so will count from 0 to
> 255 and wrap around. Once every cycle, the TRIS instruction makes the
> PORTB pins corresponding to ones in W high impedance, and the others
> low impedance. This switching can be visually observed by connecting a
> LED and a resistor to the pin.  Pin RBn will display a frequency
> F=f/4/m/2^(n+1), where f is the clock frequency, and m is the memory
> size. Eg, for a 16C84 with an RC oscillator 3.3 k/20pF, RB7 will
> display approx F = 5 M/4/1k/256 = 5 Hz, which is easily visible.  A
> broken, non-ADDLW0xFF instruction which doesn't increment W will
> typically cause at least a doubling of F. The WDT is enabled, so there
> will be "hiccups" every 2.5 s.
>
> Personally, I like the TRIS PORTB version best, but the idea can be
> varied. For instance, TRIS PORTB + SLEEP, will make an ultra-slow
> blinker program, with period 5s on RB1, 10s on RB2,... about
> 5 min on RB6, 10 min on RB 7. (Egg timer?)