gordo@athena.mit.edu (Garet G Nenninger) (04/15/88)
I'm working on a small (VERY small) microcomputer control board to do things like convert morse code and RTTY to RS-232 serial as well as FAX to printer codes. Yes, I do realize that such devices are commercially available, but it seems more fun this way. I currently use a $30 VIC-20 to do these (well, not the fax). Anyway, it looks something like: 6502 microprocessor 8 K RAM/ROM (I'll probably use a battery-backed RAM cartridge that would be used for both the program and data storage. It would be mapped at both $0000 hex (for page-0 addressing ops and stack use) as well as at the top 8K of memory (for reset vectors)) 6522 VIA for printer output, demodulator input, timer one LSTTL IC to decode the memory (probably a quad XOR; it works out well for the ghost image of the RAM at the top of memory.) Anyway, right now I'm looking at using a crystal clock oscillator instead of a crystal and some gates. However, I've seen it hinted that you can hook a crystal directly to the 6502 and get it to oscillate. Does anyone have anything on this; I have never seen anything concrete?
jsnow@esunix.UUCP (John Snow) (04/19/88)
From article <4641@bloom-beacon.MIT.EDU>, by gordo@athena.mit.edu (Garet G Nenninger): > > Anyway, right now I'm looking at using a crystal clock oscillator > instead of a crystal and some gates. However, I've seen it hinted > that you can hook a crystal directly to the 6502 and get it to > oscillate. Does anyone have anything on this; I have never seen > anything concrete? I'm looking at an old Synertek data book that shows an example of doing this very thing. It shows how to do it with a crystal, 1 inverter from a 74LS04 package, a 10pf cap, a 330K resistor, and two diodes. Let's see if I can describe how it is connected. The phase2 output of the 6502 feeds the input of the inverter. The cap goes between phase2 and phase0. Connected to phase0 are the two diodes. One diode has its cathode connected to phase0 and its anode to ground. The other diode has its anode connected to phase0 and its cathode to +5V. Also connected to phase0 are one side of the XTAL and one side of the resistor. The resistor and XTAL are in parallel and the other side of each goes to the output of the inverter. They also show the output of the inverter feeding the input of another inverter. The second inverters output is designated as the phase2 system clock to the rest of the system. I seem to remember some old Rockwell 6502 documentation that showed a simpler circuit involving a crystal. It also showed how to use just a resistor and capactor to make the 6502 oscillate if precision timing was not important. However, I haven't been able to dig up this old documentation yet. -- ----------------------------------------------------------------------------- John F. Snow UUCP: {ihnp4,decvax}!decwrl!esunix!jsnow Evans & Sutherland Computer Corp. Compuserve: 71550,1152 BIX: johnfsnow Salt Lake City, Utah