10 Mar
2005
10 Mar
'05
4:19 p.m.
I'm looking to drive both Unibus and Qbus with 5v
tolerant
Spartan2 (2.5v) fpga I/Os... anyone done this before?
Yes, using the DS3662. Unfortunately it has been discontinued, as was the
DS8641. Surplus DS8641 chips seem to be not that hard to find, I'd
probably use those.
I'v heard of people using 4000 series cmos(??) in
instead
of the approved DS8640/1, DC005, DEC9881, DEC380 etc
CD4000 CMOS drivers and receivers have nowhere near the required specs.
Spartan2 3.3V CMOS I/O is 5v tolerant without external
resistors,
FPGA port pins have nowhere near the required specs either.
but since lines may have to be series source
terminated anyways,
No. Series termination on a multidrop bus is NOT viable. You
have to use Thevenin or Thevenin-equivalent termination on the
ends of the bus only.
Spartan2e or Spartan3 might work as well..
Still nowhere near the required specs.
Ideally I'm looking
to use the same part for both Unibus and Qbus, possibly fiddling Vref.
AFAIK, if you get drivers and receivers that meet the required specs,
they will work fine for Qbus, Unibus, and Omnibus.
The required specs are:
Driver:
Vol: 0.7V max at 70 mA - rules out 4000 CMOS and all FPGA pins
leakage: 25 uA max, with Vcc from 0V to 5.25V
capacitive load: 10 pF max
rise and fall times (10%-90% voltage) must be no faster than 5 ns
Receiver:
Vil: 1.3V max
Vih: 1.7V min
capacitive load: 10 pF max
leakage: 80 uA max at 3.8Vdc, with Vcc from 0V to 5.25V
I don't know of any current production parts that meet the Qbus specs.
Most drivers don't meet the Vol spec at full current. The receiver Vil
and Vih specs are hard to meet, as they aren't typical of logic parts.
The leakage specs are also a problem for many parts. DEC used specially
selected DS8641 chips.
The closest current production chips I've found do not fully meet the
specs, but at least come close in many regards:
TI SN75138
TI AM26S10
Eric