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Driving old MOS chips: 6550 and 6540 : Current?
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NivagSwerdna:
I'm planning to build myself a small tester for 6540 MOS ROM and 6550 MOS RAM chips to diagnose a sick Pet 2001 which I recently purchased.

To this end I am building up a small board based on ATSAMC20G15A-AUT for which I have a few samples in my parts bin.  The I/O on a SAMC20G can sink 5mA and source 3mA. 

What are the actual requirements for driving these chips?  3mA will be plenty?

I'm also thinking about putting current limiting R on all pins to the ZIF socket... How do I calculate those values?  Just limit to 3mA?

Thanks in advance

PS
Power and GND will be supplied separately
greenpossum:
MOS gates are high impedance and current draw is negligible. You should be more concerned about the logic levels, whether the driver can provide adequate logic voltages, especially if you interfacing from BJT logic.
tggzzz:
My first actions would be a signal integrity test: are all the voltages within the relevant logic level spec, are the setup/hold times observed, and are transitions clean. If not then that has to be sorted out first.

Beyond that you have to define what tests you think are worth doing, especially the difference between basic functionality and parametric tests.

What drivers were used in the PET? That should give you some clue.

For DC testing I'm sure 3/5mA would be fine, but what about full speed tests? To a first approximation, CV = Q = It where C is the input capacitance, V is the voltage swing in time t. So the current to charge the load capacitance would be I=CV/t. At a guess, C=5pF (per input), V=3V (for TTL), t=5ns, giving 3mA. So, the 3/5mA output drive isn't wrong, but is close. That might matter if you are doing tight parametric tests, but is unlikely to be an issue if you are allowing plenty of margin in setup and hold times.

What benefit do you expect from a current limiting resistor?

Do also check whether the jellybean logic chips are OK. I've seen a 7404 (?07?) become faulty and fail to drive an unobtanium high voltage driver.
NivagSwerdna:

--- Quote from: tggzzz on March 18, 2020, 11:16:43 am ---What benefit do you expect from a current limiting resistor?

--- End quote ---
To protect the uP when there is a short in the DUT
tggzzz:

--- Quote from: NivagSwerdna on March 18, 2020, 11:40:35 am ---
--- Quote from: tggzzz on March 18, 2020, 11:16:43 am ---What benefit do you expect from a current limiting resistor?

--- End quote ---
To protect the uP when there is a short in the DUT

--- End quote ---

OK, but don't forget to think about the V=IR drop and how that might affect signal integrity. That will require contemplating the DUT's datasheet, of course.

You might consider doing the smoke test, then shorting out the resistors.
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