Bus contention

Often in serial applications we find a chip with a Data In and a separate Data Out pin. Many times, the Data Out pin will not be driven at the same time that the Data In pin is expecting to receive data. It is therefore possible to connect the Data In pin to the Data Out pin and use only one I/O pin on the uP to handle both jobs. There is, however, always the possibility of a 'bus conflict'.

Byron A Jeff says:

A bus conflict occurs when the pin from the PIC driving Data In and the Data Out from the EEPROM are both active at the same time. It's even worse when one is set to high and the other to low because it causes a short which of course is very very bad for the circuit.

The solution is simple. Put a current limiting resistor on Data Out so that it cannot drive the bus at full current. Conflict resolved.

I believe the last time I did this I used either a 1K or a 2.2K resistor between Data Out and the PIC. Worked like a champ.

BTW this bus conflict is always a transient event because Data Out is usually tri-stated when not in use. So it's not like the PIC will have to suck down current all the time. The conflict only exist long enough for the PIC to finish sending the command to the EEPROM, then switching to input mode to read the results.

BTW this is the reason that you limit Data Out instead of the PIC pin because you don't want data from Data Out being presented to Data In while the PIC is still clocking in a command.

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