Trivial Programmer
termination for long cables

By: Byron Jeff, developer of the
Trivial Low Voltage PIC Programmer

I'm pretty sure we all want to have longer cables to our programmers. I've been wanting to tackle this problem for a while. Well after taking a read of the Art of Electronics, I decided to take a stab at it.

The long cable (I tested with a 6ft[2m]) acts as a transmission line. It's no problem for the relatively static lines (Vdd, MCLR, LVP) and even the data line has enough time to settle before anything bad happens.

So there's really only one problematic line: the clock line. The problem is that typical reflections will cause the chip to clock multiple lines for each actual pulse. So even if the programmer is properly wired, and the software is properly configured, it still won't program.

Since I have my PIC Designer prototype running and the short cable made it difficult to place, solving the long cable priority because a priority.

I tested out a couple of different termination techniques:

• Parallel termination of a single resistor tied between the port pin and ground. The value of the resistor is supposed to match the impeadance of the cable. I tested a few different values: 1K, 100 ohms, 270 ohms, with no great success.
• I then decided to try AC termination. This adds an additional capacitor between the resistor and ground. AND IT WORKED LIKE A CHAMP! I tested with the 270 and a 1 uF electrolytic.

Give it a spin.

BAJ