Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!cbmvax!daveh From: daveh@cbmvax.UUCP (Dave Haynie) Newsgroups: comp.sys.cbm Subject: Re: 6551 interfacing Message-ID: <6925@cbmvax.UUCP> Date: 19 May 89 05:36:20 GMT References: <5944@cs.Buffalo.EDU> Organization: Commodore Technology, West Chester, PA Lines: 47 in article <5944@cs.Buffalo.EDU>, bowen@cs.Buffalo.EDU (Devon E Bowen) says: > > I'm currently working on a board for the expansion bus that will give the > 64 a 6551 UART for much higher baud rates. I finished the wiring last night, > plugged it in and it almost works. I've got the chip set up so it takes > over the I/O expansion area number 1 from 6E00 to 6EFF by running the > chip select directly to the IO1 pin on the bus. I admit it's been awhile, but aren't the I/O spaces at DExx and DFxx? In any case, if you're basing your chip select on the IO1* pin, you won't get any contention -- the mapping hardware will only generate a chip select if the I/O space is mapped in. > The problem I'm having is that sometimes it takes more than one write to > actually get the data to the chip and sometimes reading multiple times > from a chip register that should be constant gives different results. I suspect that you're having setup time problems. Basically, you can't just hook a 6526 up to the expansion bus and expect it to work OK. The chip select and addresses must be valid at least 90ns before the PHI2 clock goes high. Like I said, it's been a long time, but I think we used to use something like this for the 6551A and similar chips: +--------+ | O 4 | +---------+ | 2 | PR* |5 PHI2_C64 --+---+D Q+----- PHI2_6551 3 | |6 DOT ------+CK Q*|O---- | CL* | +---------+ 1 O 74LS74 | o +5V The idea is to delay PHI2 going into the 6551 until the addresses and chip select are valid. > Devon Bowen (KA2NRC) FAX: (716) 636-3464 > University at Buffalo BITNET: bowen@sunybcs.BITNET > Internet: bowen@cs.Buffalo.EDU > UUCP: ...!{watmath,boulder,decvax,rutgers}!sunybcs!bowen -- Dave Haynie "The 32 Bit Guy" Commodore-Amiga "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: D-DAVE H BIX: hazy Amiga -- It's not just a job, it's an obsession