High impedence versus high signal for DTACK_bar

greenspun.com : LUSENET : ece342 : One Thread

Will the bus protocol function properly if instead of driving DTACK_bar hi ('1') I set it to high impedence? That is, DTACK_bar will be either low signal ('0') or high impedence ('Z'). If not, then for how long should I keep DTACK_bar hi (at the end of read or write operation) before setting it to high impedence.

-- Rostyslav Kyrychynskyi (kyrychy@ecf.toronto.edu), January 26, 1999


On the Gizmo board internal bus (not the buffered copy available on J6) there are pull-up resistors for the open-collector, so if nobody drives it there will still be a '1'. Thus you can just assert it low from HiZ. It is better to have the default state of DTACK being '1' (in the idle state) though. Whether it will actually drive high for some period before you assert it depends on the implementation of your state machine. (You probably will turn on the OE at the same time as you drive DTACK low, so you may never see the '1'.)


-- Robin Grindley (grindley@eecg.toronto.edu), January 27, 1999.

Moderation questions? read the FAQ