Re: 6502 to 8501: new details

From: Konrad Burylo (
Date: 2002-02-23 20:39:22

At 11:05 02-02-22 +0200, you wrote:
>The NMOS (n-channel metal oxide semiconductor) chips in the Commodore can 
>actively pull signals down, but not up.  The opposite is PMOS, which is 
>quite rare, I think.  The combination of NMOS and PMOS is called CMOS 
>(complementary metal oxide semiconductor), which can drive signals both up 
>and down. [...]
>I also noticed that the 33pF capacitors in the crystal circuit are 
>unnecessary.  (How did I find this out?  I have run out of SMD caps and 
>resistors; I only had the five capacitors for the MAX232.)
>         Marko

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).
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...

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)  ;-)

Konrad Burylo

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