4 Mar
2014
4 Mar
'14
1:22 p.m.
On Tue, Mar 4, 2014 at 1:09 PM, Al Kossow <aek at bitsavers.org> wrote:
On 3/4/14 11:09 AM, Pontus Pihlgren wrote:
Unfortunately the stated input threshold calculations for the CY62256 and
74ABT245 are entirely meaningless. Having a chip with Vil and Vih spec of
0.8V and 2.2V does NOT imply that the threshold is 1.5V. I don't know
where to expect the theshold on those particular chips, but in general the
threshold for CMOS tends toward 1/2 Vcc. Obviously to guarantee Vih at
2.2V, they've biased it to move the threshold, but that doesn't necessarily
make it close to (Vil+Vih)/2.
The same reasoning applies to the 74ABT245 with Vil and Vih of 0.8V and
2.0V.
The input thresholds also vary quite a bit with temperature and supply
voltage, which is another reason why (Vil+Vih)/2 is not a reasonable
assumption.
There's a reason DEC specified the Vih of 1.4V, which is that on a heavily
loaded bus it's entirely possible for a valid logic 1 to be that low. In
1983 I tried to debug a large 8/e system which had been donated to a
community college after years of service controlling the scoreboard at
Kauffman Stadium. I often saw the logic 1 on the bus signals lower than
1.6V.
Eric
source the bus drivers
look closely at the design.