>when ports are used as input, are levels TTL or CMOS compatible ???
>
>tnx,
>Ed
The input threshold is set at TTL levels, that is, the input
switching threshold is around 1.3 volts.
Be that as it may, although standard CMOS switching levels are near
2.5 volts, and TTL switching levels are near 1.3 volts, as a
practical matter the BASIC Stamp inputs are compatible with _either_
TTL or CMOS. Don't be confused about it. Stamp inputs are in fact
hi impedance CMOS, not current-sinking TTL, which is a thing of the
past. In the early days it was necessary for CMOS chips to work with
the older TTL family. A family of CMOS (74HCT...) was developed that
had a 1.3 volt input threshold, and thus was born "TTL compatible".
The choice of threshold is kind of arbitrary as far as CMOS logic is
concerned, because CMOS outputs swing firmly from rail to rail (0 to
5 volts), unlike the older TTL logic outputs that were meant to sink
current and had trouble swinging above 3 volts.
Archiver
Posts: 46,084
Comments
>when ports are used as input, are levels TTL or CMOS compatible ???
>
>tnx,
>Ed
>
>Ed -
The I/O Ports are CMOS compatible.
>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/
>
>tnx,
>Ed
The input threshold is set at TTL levels, that is, the input
switching threshold is around 1.3 volts.
Be that as it may, although standard CMOS switching levels are near
2.5 volts, and TTL switching levels are near 1.3 volts, as a
practical matter the BASIC Stamp inputs are compatible with _either_
TTL or CMOS. Don't be confused about it. Stamp inputs are in fact
hi impedance CMOS, not current-sinking TTL, which is a thing of the
past. In the early days it was necessary for CMOS chips to work with
the older TTL family. A family of CMOS (74HCT...) was developed that
had a 1.3 volt input threshold, and thus was born "TTL compatible".
The choice of threshold is kind of arbitrary as far as CMOS logic is
concerned, because CMOS outputs swing firmly from rail to rail (0 to
5 volts), unlike the older TTL logic outputs that were meant to sink
current and had trouble swinging above 3 volts.
I hope that clarifies the issue.
-- Tracy