Re: 6522 VIA Questions

From: Ruud Baltissen (
Date: 2001-08-28 09:34:27

Hallo Nick,

1) Both the ports of the 6522 can be latched, controlled by an external
signal. This feature is used for reading data from a floppydisk by the 1541.

2) "controlled by a line from the peripheral" I'm not sure what you mean, an
external signal controls wether the data is outputted or not like a 541 or
245? If that is what you mean then: no, the 6522 is not able to do that. But
IIRC, the 8255 cannot either.

3) on FUNET you'll find some docs on the 6522 that can tell you if the
onboard shift register functions as you want to. BUT... these docs don't
mention the bug related to this shift register. A quick search didn't reveal
a doc mentioning this bug. (or am I wrong, Marko ?)

> Is either chip more attractive from a C64 interfacing point of view?

The 8255 has 24 I/O lines, the 6522 ~20. But the ones of the 6522 can
progammed individual, the ones of the 8255 only in blocks of 4 or 8.
Remark: nasty habbit of the 8255, reprogramming the direction of a block
resets all outputs :(

Both chips need some extra hardware for connecting them to a C64: the 6522 a
74 due to the timing, the 8255 a 139 due to its IORD and IOWR line.

  / __|__
 / /  |_/     Groetjes, Ruud
 \ \__|_\

       Message was sent through the cbm-hackers mailing list

Archive generated by hypermail 2.1.1.