From: Hársfalvi, Levente (
Date: 2001-03-30 16:19:05

Hi all!

I'm currently doing some timing measurements on my Commodore Plus/4.

Since I have no digital oscilloscope, I'm using an Atmel RISC
microcontroller (an AT90S2313) as an 'intelligent' replacement. This
chip is capable of running at the master clock of the Plus/4 (PAL
version, Phi0 = 17.73447 MHz) and executing one instruction per cycle
:-O so it's pretty ideal for such a purpose (still not mentioning its 16
bit timer with external capture event possibility + the onboard analog
comparator ;-) ).

Currently, Phi0 is generated by the micro. The clock is either simply
passed on, or pre-divided by 2 or 4 before passing to the 8360 TED in
the Plus/4. (Have you seen how funny a Plus/4 running at quarter speed
is? :-D). Deriving all clocks from one source makes sure that everything
is in sync with the others, and measuring the same signal with different
divider setting gives some more precision (or gives some ideas on the
real operation).

I seem to have some ideas on what to measure (should create a listing,
but until now the measurement board was not really 'safe', I was rather
experimenting...). On the other hand, I'm sure some of you have good
ideas, or things that you once wanted to measure but never got around to
do it or never had the instrument to do so. Once this thingy was built
(some of it, especially the 'divider' 74S74 with the 74F125 on the top
on it :-O - kind of spaghetti... ), I guess I'm better doing all
possibly important measurements now (since I'll probably take the
circuit apart and won't ever feel neccessary it 'enough' to put the
components together again).

So, if you're interested in some particular data, just tell me your

Current plan: measurement data of the Plus/4, a C64 and a VIC-20 (both
in PAL and NTSC modes). I also have an old CBM 4016, but I'm not really
familiar with its hardware so I'll probably leave that out, unless
someone feels it _really neccessary to join it into the group (that
case, also please give me some help on what to measure). Also, I only
have the 6567R56A from the NTSC VIC-IIs.

I'm willing to make some mesurement docs with this data, probably
appearing on Funet someday.

BTW I've just found something interesting. The sound output of the
Plus/4 is pulse width modulated, the PWM running at the dot clock of the
computer. The period is 16 or 32 dot clock cycles when both channels are
on or just one channel is on, respectively; (in the latter case, the
'second' of the two phases is kept constant 0, 'halving' the average
duty). The duty cycle grows as '1 - 3 - 5... - 15' as function of volume
level 1,2,3...8.

However, there's something strange: the 'end point' of the duty cycle is
always 'late' by the time of approximately one master clock cycle time.
This delay is constant, it is not a function of the master clock :-O. 

Check this. In NTSC mode, dot clock = master clock/2. When measuring the
PWM signal of volume level 1 ($ff11=$91), I should get '2' as duty cycle
(since my unit is the master clock). Instead, the result is '3'. If I
halve the master clock for the Plus/4, I should get two times the
previous one, '4' - I get '5' instead. When I quarter the clock, the
'theoretical' result is 8, and I get '9'. ...Should there be an internal
delay 'unit' somewhere in this part of the TED? ...It should be
something like this, but why on Earth?... 

Best regards, and tell me your ideas...


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