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Write '0's to the port, thus forcing a 1-to-0 transition that releases the weak pull-up from holding '1's at the port. Weak pull-up remains as along you are letting '1's to remain at the port.
I don't think it is possible to disable the weak pullups on this micro. It is basically an 80C32, where the pullups are always on, to allow the I/O pins to function as both inputs and outputs, simply by clearing the port latch; there is no data direction register.
Is it really that important to have the pullup disabled? Can you use a gate in front of that, to work as a buffer?
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