Re: 6502 to 8501: new details

From: Richard Atkinson (
Date: 2002-02-23 20:54:57

On Sat, 23 Feb 2002, Konrad Burylo wrote:

> To make it clear: NMOS technology use two kinds of N-channel transistors:
> enhancement mode (normally off) and depletion mode ones (normally on). CMOS
> technology use enhancement mode N-channel and  P-channel (instead of
> depletion mode N-channel).

Depletion mode transistors are what I'd call normally "half on". It is
possible to turn them off with a negative Vgs and to turn them on more
with a postive Vgs, but they are typically used with Vgs=0V as resistive

> I haven't heard of PMOS logic chips - but the hole mobility is about 2.5
> times less than electron mobility and P-channel transistor should have ~2.5
> times wider channel to have the same transconductance as N-channel one. So
> PMOS gates would simply be bigger...

Or slower. In fact, generally not as good. Does anyone know of any PMOS
logic ICs? I was surprised to find out recently that the SN76489 DCSG used
in a number of old systems is in fact bipolar - current injection logic if
I'm not mistaken.

> Marko - have you tried to exchange the crystal with a different one ?  I'm
> curious if it's not just an accident, that oscillator runs without
> capacitors. Once I was doing something with AT-mega, and it didn't work
> without capacitors. Also didn't work with 100nF one (a guy put one 33pF and
> one 100nF by a mistake)  ;-)

If those capacitors are doing what I think they are doing, they are there
to improve the start-up time. If this isn't important in your circuit,
then by all means leave them out.


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