VIC-20 schematic amazing coworkers

[nippur72]

today I replaced the picture on the wall of my office with a big A2 print of the VIC-20 circuit diagram (actually two joints A3 sheets).

To my amusement, all my co-workers stepping into my office are stopping in front of it all astonished. Some dare to ask, so I start to explain: "oh well, this is the processor 6502... this is the rom... this the ram..." and so on. Sometimes I adventure explaining the reset circuitry, how it sends a reset signal directly into the CPU and the VIA chips. It is very suggestive to people.

And then everybody agree on the amazing simplicity of those old machines. Ah! the old good times, when computers where real computers!


(P.S. BTW, I've still to understand why there are two 6522 and where is the clock oscillator for the CPU--I see only a 14 Mhz clock for the video chip).

[carlsson]

Umm.. the two 6522 handle different tasks but I thought you were already aware of that. You have keyboard, joystick, tape, floppy and userport to handle while each 6522 only has eight pins of I/O.

[eslapion]

(P.S. BTW, I've still to understand why there are two 6522 and where is the clock oscillator for the CPU--I see only a 14 Mhz clock for the video chip).

The 6560 and 6502 are synchroneous. The 6560 access the bus on half the cycle and the 6502 on the other half.

The 14.31818MHz (different for PAL) clock is divided by 14 by a small logic chip close to the 6560, giving a CPU clock slightly higher than 1MHz.

[eslapion]

...each 6522 only has eight pins of I/O.

I believe they have at least 16.

[nippur72]

The 14.31818MHz (different for PAL) clock is divided by 14 by a small logic chip close to the 6560, giving a CPU clock slightly higher than 1MHz.

Is it the one connected to the pin 35 of 6560? In the schematic I have I miss the number of the IC. Do you know what is it?

[carlsson]

True, I forgot each VIA has two ports. Still, I wonder if that was Nippur's question.