From: Jim Brain (brain_at_jbrain.com)
Date: 2007-05-09 17:00:47
Marko Mäkelä wrote: > On Wed, May 09, 2007 at 01:56:20AM -0500, Jim Brain wrote: > >> LO->HI: switch DDR to input, then switch logic level to high to engage >> pullup. Doing it this way ensures you never drive pin with 5v directly >> HI->LO: disengage pullup, then switch DDR to output. >> > > Oh, I did remember something about setting the PIN bit on the AVR > when composing my answer. That was for engaging the pullup resistor > when outputting a logic 1. But I don't think it should be necessary > for the joystick port, because mechanical joysticks do not contain any > pullup resistors. And I think you can safely leave the PIN bit > initialized to 1 at all times. > I think you mean 0. PORT=0, DDR=0, input, no pullup PORT=0, DDR=1, output, low In my defense, I wrote the joystick code after the initial IEC code, so I was in IEC mode when writing it. > It's a different matter on a shared bus, such as the Commodore serial bus. > There you might want to disengage the pullups when the device is not active, > or you won't be able to attach very many devices to the bus. I remember > that George Page, who parted with his collection some years ago, once tested > how many devices he can connect to the serial bus. I think he was getting > problems after some 5 devices. That might be because the Commodore > devices are equipped with fixed (passive) pull-up resistors. > That's a good idea, I'll add that to the tweaks list. 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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