No load resistor

Community General Discussions
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programming_header.jpg (36615Bytes)
I’m working with a psoc micro and many of the schematics they provide have “no load resistors”. I was just wondering if anyone here knew what a no load resistor is?

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The only way a resistor
The only way a resistor could provide zero load was if the resistance was effectively infinite, as in an open circuit.
I can’t say for sure without seeing the schematics myself, but I’d suspect they’re actually saying that there are not any ‘load resistors’ shown, a load resistor being a resistor that just soaks up a set amount of power.

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I attached the schematic to
I attached the schematic to the orignal post.

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Haha, they’ve got a 'zero’

Haha, they’ve got a ‘zero’ resistor on there too. Where I come from, we call that a ‘wire’.

Anyway, it seems in this case that they want you to use a resistor that’s large enough for there to be (virtually) no load on the TMS and TDO lines. Basically just a very weak pull-up resistor.
My recommendation would be to use a resistor of around 100kΩ, for a worst-case load current of 50μA @ 5V supply. Normally I’d go for 20kΩ, but I don’t know how sensitive this IC is so you might as well be extra gentle on it. You could go for an even higher resistance, but that would slow down the IC’s response on those pins, plus it doesn’t guard against interference as effectively.

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Great! I’ve asked this
Great! I’ve asked this elsewhere and I now have exact opposite responces. one professor at school thought it should be a near zero resistance. I’m thinking that you are correct in it being a very large resistance which would result in a very small resistance.

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My thinking was this: when a

My thinking was this: when a circuit is ‘loaded’, it means power is being drawn from it. For this reason, dummy loads such as the ones used for testing radio equipment or power supplies are normally rated primarily by the amount of power they can take.
P = VI
V = IR -> I=V/R
p = (V/R)*V
p = V²/R

The maximum voltage dropped across the resistor will be the Vdd supply voltage, which is a nice constant, leaving us with a relationship that states that as the resistance drops, power will increase. A resistor of nearly zero resistance will consume as much power as you can feed it. Not exactly what you’d expect for a ‘no load’ resistor =)

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thanks, I agree with what

thanks, I agree with what you just said. now the question is why bother including such a large resistor? It looks like I need to find some documentation on the programming of the PSoC microcontroller.