Electronic – JTAG Clock line Termination

jtagtermination

The performance of my self-made JTAG cable isn't great, I can clock it up to 1.5Mhz max. I've read that terminating the clock line is a thing so I investigated:

  • One solution suggested to add a series termination resistor of 220 Ohm (or so).. Haven't tried that yet, but it makes sense.

  • Another solution recommended this:

schematic

simulate this circuit – Schematic created using CircuitLab

That makes perfect sense to me. It's like ordinary termination with a high-pass built in.

Something makes me wonder however:

The capacitor isolates from the actual DC level, so it should not matter if I connect it to ground, 1V, 10V or whatever as long as 'whatever' is of low impedance, right?

Could I connect the capacitor to – lets say – my 3.3V CPU supply instead of ground or is that a terrible idea?

Reason I ask is, that on my cable the clock line is directly beside the 3.3V so decoupling against that reference would fit better.

Best Answer

There are 2 reasons for using an RC end termination topology. (can only be used on signals with 50% duty cycle such as - clocks)

  1. Reduce power consumption
  2. Centering the eye diagram.

If you do not care about either of these then a single far end resistor will do. (screw the cap) and you can end the resistor to either Vcc or Gnd.

If you do care, then connecting the capacitor to either Gnd or Vcc should be fine. The capacitor will appear to be a short circuit during the edges, which is what you want and the average voltage between the capacitor and the resistor would be 1/2Vcc, which reduces your power.