Re: Static PET video timing?

Am 29.03.2023 um 13:08 schrieb Rhialto:
> On Tue 28 Mar 2023 at 23:49:02 +0200, afachat_at_gmx.de wrote:
>> Hi there,
>>
>> has anyone actually ever analyzed the static (or even early dynamic PET) video
>> timings? The ones where the video display is created by discrete hardware?
>  From a practical software point of view but not from hardware, which is
> also interesting.
Apart from what Olaf was writing there is also the demo "A Bright 
Shining Star" for CRTC-less PETs which for it's PECBM-mode uses the same 
technique as in the HI-RES program to provide pseudo hi-res graphics of 
10 chars width by updating the video-RAM on the fly:

https://www.pouet.net/prod.php?which=91735

I do not own an original PET, only a Mini-PET by TFW8B which strives to 
emulate (most) of the original PET-series, but in regards to video is 
closest to the CRTC-less PETs. However both demos mentioned above won't 
work correctly. I could hack the Cursor-demo to work to some degree. 
Effectively the timings are obviously different from an original PET. 
However, since it reports the VBLANK on $e840 same as the original I 
hacked together a quick program that counts the cycles between the 
VBLANK-bit switches over 256 tries each and averages those out with a 
small BASIC-program:

https://www.tokra.de/pet/petcycle.zip

Source is included. I'm not sure I'm getting the timer-values 100% 
correct, I have not used timers before really. There is bound to be some 
jitter of 4-14 cycles(?) by polling $e840 twice for each measurement. 
And I'm not sure what to deduct for setup and storing of the 
timer-values (i.e. how many cycles does starting the timer and loading 
the value from it cost). Anyway, this should give some info over the 
number of cycles per frame.

Torsten