MOS Technology 6522

The 6522 Versatile Interface Adapter (VIA) was an integrated circuit made by MOS Technology, as well as second sources including Rockwell and Synertek. It served as a I/O port controller for the 6502 family of microprocessors, providing the parallel I/O capabilities of the PIA as well as timers and a shift register for serial communications. The 6522 was very popular in computers of the 1980s, particularly Commodore's machines, and was also a central part of the designs of the Apple III, BBC Microcomputer and Apple Macintosh.

Input/output ports

The VIA contains 20 I/O lines, which are organised into 2 8-bit bidirectional ports (or 16 general-purpose I/O lines) and 4 control lines (for handshaking and interrupt generation). The directions for all 16 general lines (PA0-7, PB0-7) can be programmed independently. The control lines can be programmed to generate an interrupt when activated (all four), latch the corresponding I/O port (CA1 and CB1), automatically generate handshaking signals for devices on the I/O ports, or output a plain High or Low signal.

Timers

The VIA provides 2 16-bit timer/counters, which can be used in one-shot (monostable) mode, free-running (divider) mode or "pulse counting" mode, where the timer will monitor the 7th bit (PA6 or PB6) on its respective I/O port, and count how many state transitions pass by. Each timer can generate an interrupt when it reaches zero, and can also output square waves on the 8th bit of its respective I/O port (pin PA7 or PB7).

Shift register

The VIA's shift register is bidirectional, 8 bits wide, and can run from either a timer-generated clock (from timer 2), the CPU clock, or an external source on line CB1. The serial input/output is on line CB2, and CB1 can also be programmed to output a bit clock for external devices. The infamous serial shift register bug is fixed by the CMD G65SC22.

Bugs

Aside from the aforementioned shift register bug, there was a potential register corruption problem that usually only occurred in systems using the 6522 with a processor having a non-6502-like bus, such as a Motorola 68000. If the address lines changed while chip select was inactive but the phase 2 clock input was high (active), register contents could be changed despite chip select being inactive. This was fixed in some but not all of the CMOS versions.