Re: 6522 VIA inputs

From: Pasi 'A1bert' Ojala <>
Date: Fri, 21 Apr 2017 13:23:31 +0300
Message-ID: <>
You can probably use a multimeter to measure the resistance from the pin 
to GND and to VDD to support or disprove the hypothesis.

Using the diode measurement (to GND and VDD) you could also probably 
find protection diodes.

It's of course possible the output driver is push-pull, but has 
sufficient leakage for it to float high when in high-impedance (input) mode.

On 21.04.2017 13:11, wrote:
>> On 2017-04-21, at 11:47, Pasi 'A1bert' Ojala <> wrote:
>> CMOS uses push-pull drives, but my impression is that older technologies use pull-ups with open-collector/open-drain drives. I.e. passive up, active down.
>> CMOS can emulate open-drain drive by using an external pull-up resistor and switching between input (high-impedance) and drive-0 states. I have used this often to create I2C using regular IO pins.
>> Microcontrollers often have weak pull-downs or pull-ups. However, resistors in the MOhm range increase the production testing time due to the requirement to measure very small currents, which is a reason they are not always used. (In addition to having a minimal effect in leakage during low-power modes.)
> Pasi, thanks - that's the very reason I am looking closer at it! I want to output data to VIA port pins acting as inputs and started to wonder whether I should drive the lines HI (seems to work in tests but if the lines are in fact pulled up then it's presumably unneeded/undesirable) or rather tri-state my outputs instead.

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Received on 2017-04-21 11:02:11

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