MOS Technology 6522

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MOS Technology 6522

The 6522 Versatile Interface Adapter (VIA) was an integrated circuit

Integrated circuit

In electronics, an integrated circuit is a miniaturized electronic circuit that has been manufactured in the surface of a thin Wafer of semiconductor material....
made by MOS Technology

MOS Technology

MOS Technology, Inc., also known as CSG , was a integrated circuit design and Semiconductor device fabrication company based in Norristown, Pennsylvania, in the United States....
, as well as second source

Second source

In the electronics industry, a second source is a company that is licensed to manufacture and sell components originally designed by another company ....
s including Rockwell

Rockwell

Rockwell can refer to:...
and Synertek

Synertek

Synertek, Inc. was an United States semiconductor manufacturer founded in 1973. The initial founding group consisted of Bob Schreiner , Dan Floyd, Zvi Grinfas, Jack Baletto, and Gunnar Wetlesen....
. It served as a I/O port controller for the 6502

MOS Technology 6502

The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured central processing unit on the market by a considerable margin, costing less than one-sixth the price of competing designs from larger companies such...
family of microprocessors, providing the parallel I/O capabilities of the PIA

Peripheral Interface Adapter

The Peripheral Interface Adapter is a peripheral integrated circuit providing parallel I/O interfacing capability for microprocessor systems. Common PIAs include the Motorola MC6820 and MC6821, and the MOS Technology MCS6520, all of which are functionally identical but have slightly different electrical characteristics....
as well as timer

Timer

A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass....
s and a shift register

Shift register

In digital circuits, a shift register is a group of flip-flop s set up in a linear fashion which have their inputs and outputs connected together in such a way that the data is shifted down the line when the circuit is activated....
for serial communications.

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Encyclopedia

Rockwell R6522p Versatile Interface Adapter

The 6522 Versatile Interface Adapter (VIA) was an integrated circuit

Integrated circuit

In electronics, an integrated circuit is a miniaturized electronic circuit that has been manufactured in the surface of a thin Wafer of semiconductor material....
made by MOS Technology

MOS Technology

Second source

In the electronics industry, a second source is a company that is licensed to manufacture and sell components originally designed by another company ....
s including Rockwell

Rockwell

Rockwell can refer to:...
and Synertek

Synertek

MOS Technology 6502

Peripheral Interface Adapter

Timer

Shift register

Commodore International

Commodore, the commonly used name for Commodore International, was a United States electronics company based in West Chester, Pennsylvania which was a vital player in the home computer/personal computer field in the 1980s....
's machines, and was also a central part of the designs of the Apple III

Apple III

The Apple III was a personal computer aimed at business users, manufactured and sold by Apple Inc. from May, 1980 until its discontinuation on April 24, 1984....
, 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

Handshaking

In information technology, telecommunications, and related fields, handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins....
and interrupt

Interrupt

In computing, an interrupt is an asynchronous communication signal from hardware indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
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 wave

Square wave

A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels....
s 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

Motorola 68000

The Motorola 68000 is a 16/32-bit Complex instruction set computer microprocessor core designed and marketed by Freescale Semiconductor ....
. 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.