BS2 to CMOS chips
Archiver
Posts: 46,084
I am interfacig a BS2 to a UCN5833A which is a BiMOS chip (not sure
how that is different from CMOS). In the datasheet the recommend
putting a pulldown resistor at the input when interfacing to TTL.
Could anyone explain what that means and why?
many thanks
Al
how that is different from CMOS). In the datasheet the recommend
putting a pulldown resistor at the input when interfacing to TTL.
Could anyone explain what that means and why?
many thanks
Al
Comments
I am interfacig a BS2 to a UCN5833A which is a BiMOS chip (not sure
how that is different from CMOS). In the datasheet the recommend
putting a pulldown resistor at the input when interfacing to TTL.
Could anyone explain what that means and why?
many thanks
Al,
Pulldown resistors are usually employed on the input of logic devices. It
ensures noise doesn't cause them to erroneously change state. Say for instance
you have an input on a logic device which needs to be at a logical low for most
of the time you would put a 470k resistor between it and ground to ensure that
any noise present on the input would not change the state of the chip. If a
(valid) logical high (5V) were present on the BiCMOS chip that 470k resistor
would appear to do nothing. Which is a good thing!
-Randy
To UNSUBSCRIBE, just send mail to:
basicstamps-unsubscribe@yahoogroups.com
from the same email address that you subscribed. Text in the Subject and Body of
the message will be ignored.
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
Do You Yahoo!?
Yahoo! Games - play chess, backgammon, pool and more
[noparse][[/noparse]Non-text portions of this message have been removed]
>how that is different from CMOS). In the datasheet the recommend
>putting a pulldown resistor at the input when interfacing to TTL.
>Could anyone explain what that means and why?
>many thanks
>Al
Hi Al,
BiMOS is a method of integrated circuit fabrication that includes on
the same device both bipolar transistors (for the output drive) and
also CMOS (for the inputs and for the logic latches). It was a big
deal when it first came out 20 or so years ago, as the two types of
fab were thought to be incompatible.
My guess would be that it should say "pullup" resistor, not
"pulldown". (??) TTL is (or, "was") a "current sinking" logic family
that was very good at pulling its outputs to low levels, but not very
good at pulling them high. The interface from TTL outputs to CMOS
inputs calls for a pullup resistor to assure that the voltage will go
firmly above the CMOS logic threshold of Vdd/2.
It is unlikely that you will be driving this chip with real TTL. You
might be driving it with "TTL compatible" logic, but that is a
different thing. It is capable of sinking enough current to satisfy
an old TTL gate, but it is also usually capable of full CMOS drive
without a pullup. Is that confusing enough?
-- regards,
Tracy Allen
electronically monitored ecosystems
mailto:tracy@e...
http://www.emesystems.com