Re: 6522 VIA inputs

From: Pasi 'A1bert' Ojala <>
Date: Fri, 21 Apr 2017 12:47:21 +0300
Message-ID: <>
On 21.04.2017 12:34, wrote:
> Although it's caption says: "Port B Output Circuit", which originally 
> made me believe that it's only the part used when the pin is 
> configured as output, it also contains a line described as "INPUT 
> DATA". This line seems to be in fact pulled up. I assume this is what 
> makes it reliable - a kind of internal pullup. Would still appreciate 
> corrections if I am missing something here.

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.)


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Received on 2017-04-21 10:03:07

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