base:cpu_clocking
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— | base:cpu_clocking [2015-04-17 04:31] (current) – created - external edit 127.0.0.1 | ||
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+ | ====== Clock Frequency ====== | ||
+ | All clock frequencies in the C64 are derived from a single clock quartz which has the frequency of 4 times the frequency of the color carrier used for PAL or NTSC. | ||
+ | |||
+ | PAL C64 master clock: 17.734475 MHz | ||
+ | |||
+ | NTSC C64 master clock: 14.31818 MHz | ||
+ | |||
+ | The CPU frequency is then calculated from that by simply dividing the frequency by 18 (PAL) or 14 (NTSC). | ||
+ | The VIC-II runs at a frequency which is exactly 8 times that of the CPU. This is the so called "dot clock" which has to be very precise in order to keep the right timing needed to generate a video signal compatible with all TVs. The CPU of the time could not go that fast, max. 1MHz, but the CPU still needs to be phase synchronous to the VIC-II because they share control of the address/ | ||
+ | |||
+ | < | ||
+ | CLOCK_PAL | ||
+ | CLOCK_NTSC = 1022727 Hz | ||
+ | |||
+ | CLOCK_VICII_PAL = 7881984 Hz | ||
+ | CLOCK_VICII_NTSC = 8181816 Hz | ||
+ | </ | ||
+ | |||
+ | The service manual (march 1992 original from Commodore, part number 314001-03) explains the clock circuitry the following way, using an NTSC machine as a reference. This is valid also for the PAL version when the appropriate differences in the figures are considered. | ||
+ | |||
+ | << Crystal Y1 develops a 14.31818MHz fundamental frequency clock signal. U31 is a Dual Voltage Controlled Oscillator. The output on pin 10 is a 14.31818 MHz clock signal called the color clock. R27 can be adjusted to obtain exact output frequency. U30 is a frequency divider that outputs a 2MHz signal on pin 6. U29 is a D flip flop which outputs a 1 MHz signal on pin 9. U32 is a Phase/ | ||
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+ | In later models of the C64 (starting from version B, PCB ASSY#250425 Schematic #251469) the above gets integrated into a single chip labelled 8701, designed and manufactured on purpose by MOS technology. The description in this case is much more simple: | ||
+ | |||
+ | << Crystal Y1 develops the fundamental 16MHz clock signal. U31 is a Clock Generator IC that outputs the 8.1818MHz DOT clock on pin 6, and the 14.31818 MHz color clock on pin 8. >> | ||
+ | |||
+ | Note by Karoshier: my C64 schematic (1982 original taken from the programmer' |
base/cpu_clocking.txt · Last modified: 2015-04-17 04:31 by 127.0.0.1