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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.

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  • \$\begingroup\$ What's the edge rate of your TCK driver? What's the overall length of the cable? What you've drawn there is known as AC termination, and for a signal like a clock with a roughly 50% energetic content, it makes perfect sense. Other common choices are series termination, and parallel termination -- Howard Johnson's High Speed Digital Design is a good resource for this. \$\endgroup\$ – Krunal Desai Mar 16 '16 at 0:09
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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.

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  • \$\begingroup\$ Thanks for the answer. I added the termination (with and without cap) and was able to bump up my JTAG clock-speed by another 150khz. Seems like termination is not that much of a problem for 15 cm cable length. \$\endgroup\$ – Nils Pipenbrinck Mar 16 '16 at 16:09
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It can reduce the impedance of the transmission line. In AC signals, the conductor behaves as transmission line. The impedance of a line in AC mode is \$j\omega L\$. It will increase as much as the frequency increase plus a \$2\pi\$ factor. We can decrease this impedance effect by adding an AC resistor in parallel to the line, i.e. by adding an RC series whose impedance is \$R+1/(j\omega C)\$. Then we have the following circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

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