kp74615@nokikana.tut.fi (Karri Tapani Palovuori) (11/07/90)
Hi, My Technical Reference Manual (B2000) claims (on page 82) that 'the signal's (=DOE) timing changes from read cycle to write cycle'. During read cycles it enables PIC's data drivers but what about write cycles? Could I use it as 'data on bus is correct' -signal during processor writes? Karri
daveh@cbmvax.commodore.com (Dave Haynie) (11/08/90)
In article <1990Nov7.085624.23235@funet.fi> kp74615@nokikana.tut.fi (Karri Tapani Palovuori) writes: >My Technical Reference Manual (B2000) claims (on page 82) that >'the signal's (=DOE) timing changes from read cycle to write cycle'. And I wrote that, didn't I. Silly me, that doesn't really say anything of much value. >During read cycles it enables PIC's data drivers but what about >write cycles? >Could I use it as 'data on bus is correct' -signal during processor >writes? I'm not sure if it would work for that, but it's not recommended. DOE is supposed to indicate when a device may start to drive data onto the bus; it's basically an indication of the start of S4, in 68000-speak. A bus master _may_ indeed legally drive data prior to DOE, but you can't be sure about the data being valid at least until UDS* and/or LDS* are driven, sometime during the S4 state, with a setup time defined by the 68000 specs as at least 40ns. Since a state is 70ns long, and TYPICAL 68000 timing has UDS*/LDS* drop by the middle of S4, it may appear that DOE would be a good data latch. But it's not guaranteed. >Karri -- Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy Standing on the shoulders of giants leaves me cold -REM