From: Jim Brain (brain_at_jbrain.com)
Date: 2007-05-09 19:07:42
Marko Mäkelä wrote: > I think you confused PIN and PORT. I didn't say anything about PORT, > and I previously wrote that you should set PORT=0 for open-collector > operation. We have the following truth table: > > PORT=0,PIN=0,DDR=0: input, high-Z ("disconnected") > PORT=0,PIN=0,DDR=1: output 0 > PORT=0,PIN=1,DDR=0: output weak 1 with a pull-up resistor > PORT=0,PIN=1,DDR=1: output 0 > > The first line is equivalent to outputting 1 if there > is a pull-up resistor somewhere else in the circuit, like I believe > there is in the joystick port. > Can you point to some documentation, as I just checked the AVR datasheet, and PIN is read-only: "Three I/O memory address locations are allocated for each port, one each for the Data Register – PORTx, Data Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins I/O location is read only, while the Data Register and the Data Direction Register are read/write." As well, the pullup is only active when PORT is HI and DDR is LO: "If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as an output pin. The port pins are tri-stated when a reset condition becomes active, even if no clocks are running." PORT=0,DDR=0 is input, HiZ PORT=1,DDR=0 is input, pullup PORT=X, DDR=1, X is output. Are you talking about a different uC, or the some MOS part? Jim -- Jim Brain, Brain Innovations (X) email@example.com Dabbling in WWW, Embedded Systems, Old CBM computers, and Good Times! Home: http://www.jbrain.com Message was sent through the cbm-hackers mailing list
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