From: Christian Johansson (c64_at_comhem.se)
Date: 2004-06-30 22:03:04
----- Original Message ----- From: "Marko Mäkelä" <firstname.lastname@example.org> To: <email@example.com> Sent: Monday, June 21, 2004 8:12 PM Subject: Re: RS-232 on VIC-20 > On Mon, Jun 21, 2004 at 07:51:54PM +0200, Christian Johansson wrote: > > > Well, actually, the biggest problem is badlines - and they're 40 us > > > long even in 2 MHz mode. In 80 column mode with the VIC screen closed > > > it's possible to use 9600 bps though. > > > > I'm not very familiar with badlines. Does that problem exist even if I blank > > the screen in 40-column mode? > > 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. Maybe MagerValp was thinking the TED, which doesn't show garbage on > the screen, but slows down the processor to 1 MHz during graphics DMA. The > TED also has two badlines for every line of text. > [CJ] 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? > > What I want to know is if the Hug routine I posted in my > > previous email could perhaps be used to get at least 4800 bps by making some > > small modifications to it. > > 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. [CJ] 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)? [CJ] 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 > > Marko > > Message was sent through the cbm-hackers mailing list Message was sent through the cbm-hackers mailing list
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