Apr
9
Drean Commodore 64 (Correction)
A while back, i talked about the PAL-N VIC-II present on the Argentinean license 'built' C-64s.
But, thanks to Juan Carlos Fekete and electroeject, members of Retrocomputacion.com forum, I realized that my calculations were wrong.
I assumed that the clock circuitry is working in PAL mode, which means that the crystal frecuency gets divided by 18 to get the processor clock frequency. In fact, the clock circuitry is working in NTSC mode: The crystal frequency gets divided by 14 to get the processor clock frequency.
Thus, according to the previous calculations the crystal frequency must be:
WRONG!
Second mistake, I didn't noticed that the colorburst carrier frequency is the crystal frequency divided by 4.
The colorburst carrier frequency for PAL-N is 3.582056MHz
But, thanks to Juan Carlos Fekete and electroeject, members of Retrocomputacion.com forum, I realized that my calculations were wrong.
I assumed that the clock circuitry is working in PAL mode, which means that the crystal frecuency gets divided by 18 to get the processor clock frequency. In fact, the clock circuitry is working in NTSC mode: The crystal frequency gets divided by 14 to get the processor clock frequency.
Thus, according to the previous calculations the crystal frequency must be:
Processor Clock = 1.014 MHz
Crystal frequency = 1.014 MHz * 14 = 14.196 MHz
Crystal frequency = 1.014 MHz * 14 = 14.196 MHz
WRONG!
Second mistake, I didn't noticed that the colorburst carrier frequency is the crystal frequency divided by 4.
The colorburst carrier frequency for PAL-N is 3.582056MHz
3.582056MHz * 4 = 14.328224 MHz
This is confirmed by this picture by Juan Carlos Fekete of a rev.A motherboard (note the 6572R1): Knowing the crystal frequency, we can now calculate the processor clock frequency:
Processor Clock = 14.3282 MHz / 14 = 1.02344 MHz
That's a little faster than a NTSC machine (1.02227 MHz).
Processor Clock = 14.3282 MHz / 14 = 1.02344 MHz
That's a little faster than a NTSC machine (1.02227 MHz).
