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Topic: TARGET_PRESENT LED fix (v2 prototype) (Read 2966 times) previous topic - next topic

TARGET_PRESENT LED fix (v2 prototype)

[quote author="ian"]

It looks like it has a lot of elements of our design, even the exact same (broken) TARGET_PRESENT circuit :)
[/quote]

We just need to add a diode in series with the CPLD pin and it would work.

Jan

Re: Forked FT2232H + CPLD design

Reply #1
I thought it was coming from the powr supply pin, not the connected pin. If we put a diode on that pin the voltage will drop .6, or maybe you mean to drop to the transistor, that could work.

That might not be the pro blem though. The inputs by default have pin keepers, an issue robots pointed out in another thread. Maybe with those disabled it will work better. I doubt it though.
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Re: Forked FT2232H + CPLD design

Reply #2
Right now, the CPLD pin is providing a weak connection to ground, causing current to flow through the LED, bypassing the transistor. By adding a diode between CPLD pin and the collector (like this: CPLD_Pin--->|----collector) this path is blocked and the LED will only light up when a target is connected.

Jan

Re: Forked FT2232H + CPLD design

Reply #3
I tested this by drilling the trace from the CPLD to the transistor to remove it. It did change the behaviour:
OLD) LED on dimly until target attached, then on bright forever
NEW) LED off until target attached, then on bright forever

I think it is coming from the bank power supply pin. I read 0.62 volts at the transistor base (target power supply) after power on.
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Re: Forked FT2232H + CPLD design

Reply #4
[quote author="ian"]
I tested this by drilling the trace from the CPLD to the transistor to remove it. It did change the behaviour:
OLD) LED on dimly until target attached, then on bright forever
NEW) LED off until target attached, then on bright forever

I think it is coming from the bank power supply pin. I read 0.62 volts at the transistor base (target power supply) after power on.
[/quote]

That sounds a lot better.

The base/emitter resistor is too large, change it from 100 k to 10k, the base resistor  should be increased to 22k.

Jan

Re: Forked FT2232H + CPLD design

Reply #5
Jan - thank you so much for the excellent debugging support. Now it works great.  I replaced base/emitter R with 10K. I only had a 20K in 0805 without getting into the R kit, so I used that as the base resistor.

Do you think we still need the diode on the CPLD pin? I'll restore the trace and test it.

Thanks for your help in this thread. I'm going to split it into a separate topic so it is easy to refer back to.
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Re: TARGET_PRESENT LED fix (v2 prototype)

Reply #6
I'm glad to hear, that it works now. :)

Without the diode the bus keepers or the input protection network of the cpld may sink current. I'd leave the diode in.

Jan