From: Marko Mäkelä (marko.makela_at_hut.fi)
Date: 2004-06-30 23:35:55
On Wed, Jun 30, 2004 at 10:03:04PM +0200, Christian Johansson wrote: > > No. In 2 MHz mode, there are no badlines, but the system will run at 1 MHz > > during memory refresh (5 slow cycles on every rasterline), making the total > > number of cycles per raster line equal 63*2-5 on PAL systems and 65*2-5 on > > NTSC. > Does this only apply if the 40-column screen is used in 2 MHz mode, > i.e. the system runs at 2 MHz all the time if using 80-column mode in 2 MHz > mode? The VIC-IIe can never be fully disabled from the C128. It generates the processor clock and produces the 40-column video signal all the time. I'm not aware of a way to disable the 5 memory refresh cycles per raster line. (Note that the VDC raster lines are not synchronized with the VIC-IIe raster lines, as the two video chips use different dot clock sources.) > > You may need to take the DRAM refresh cycles into consideration. The bit > time > > for 4800 bps is 17734472/18/4800 = 205 slow cycles on the PAL C128 and > > 14318181/14/4800 = 213 slow cycles on the NTSC C128. Okay, maybe you can > > ignore the DRAM refresh even at 9600 bps. > > Forgive me my ignorance but does this mean that the system clock > frequency on a PAL machine is derived from an oscillator running at 17734472 > Hz that is divided by 18 by a clock divider (I guess this is described > somewhere in the C64 Programmer's Reference Guide but I can't find it right > now)? It's not described in the Programmer's Reference Guide, but you have to deduce it from the schematic diagram. Be sure not to use the PRG's buggy schematic diagram, but a more accurate one, e.g., from http://www.funet.fi/pub/cbm/schematics/. > I think that your calculations may not be correct for the Hug routine. > These are the values used by that routine and they are different from what I > get with your calculations: > > ; Start bit times. > ; > strt24 .word 459 ; 2400 > strt12 .word 1090 ; 1200 > strt03 .word 4915 ; 300 > ; > ; Full bit times. > ; > full24 .word 421 ; 2400 > full12 .word 845 ; 1200 > full03 .word 3410 ; 300 If you multiply the above figures, e.g., 421*2400, you should get the processor clock speed in Hz. Most figures are around 1.1 MHz. The PAL VIC-20 clock speed is 17734472 Hz/16 (and not /18 as on the C64 and C128), or 1108404 Hz. So, the figures look quite okay. On the PAL C64 and C128, you should use 985248 Hz as the base value. If I remember correctly, the NTSC VIC-20 runs at the same clock speed as the NTSC C64 and C128, 14318181 Hz/14=1022727 Hz. Marko Message was sent through the cbm-hackers mailing list
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