12 votes
Accepted

What is the difference between the 6522 VIA and a UART?

First of all, the 6522 has some things (parallel ports and timers) that a UART simply doesn't have at all. A UART will normally generate some programmable number of start bits, stop bits, and possibly ...
  • 3,892
12 votes
Accepted

Why does the Vic-20 have two VIA chips?

[Not sure if I understand the question in full] A parallel port chip like the 6522 is about providing port lines. In case of a 6522 these are 16 lines (two 8 bit ports) for arbitrary use plus 4 ...
  • 178k
9 votes
Accepted

Why did the Vic-20 need special chips for I/O?

Commodore owned MOS technologies, who made the VIA chips. Although the VIC-20 might have been able to replace a VIA chip with a 4051, a 74LS139, a couple of 74LS373s, and eight resistors, I don't ...
  • 29.8k
8 votes

Could the Commodore 64 have used the 6522 VIA instead of the 6526 CIA?

Preface: Speculations of an alternate history are unfit for RC.SE, so this answer is restricted to fact checking. TL;DR; From a software (Kernal) PoV there is no reason for using a 6526, but there are ...
  • 178k
7 votes

Comprehensive test suites for MOS 6520 PIA / 6522 VIA / 6526 CIA

Klaus Dorman's functional tests for the 6502 are fantastic: https://github.com/Klaus2m5/6502_65C02_functional_tests
  • 965
6 votes

Why did the Vic-20 need special chips for I/O?

Sinclair was a company particularly sensitive to BOM¹ costs. They were willing to put in quite a lot of development effort to have a machine that was a cheap as possible to manufacture, and selling ...
  • 22.2k
6 votes

What is the difference between the 6522 VIA and a UART?

TL;DR: UART is a generic term for a serial interface function (chip) for asynchronous transmission. In contrast 6522 is a specific chip - which does not include UART functionality. Implementation for ...
  • 178k
3 votes

What is the advantage of using CB2 over CA2 on the PET user port?

It's as usualy a combination of needs and possibilities within restricted resources that have to be weighted to generate best usability. Need for a port-bit to switch graphics Like of simple port use ...
  • 178k
3 votes

Comprehensive test suites for MOS 6520 PIA / 6522 VIA / 6526 CIA

I am unaware of any formalised test suite for either the 6520 or 6522. The 6526 is at least partially covered by Wolfgang Lorenz's C64 test suite. Because of the applicability to other machines, you ...
  • 32.6k
2 votes

What is the difference between the 6522 VIA and a UART?

Referring specifically to the serial communication functionality of the VIA, there is a very crucial difference between that and a UART. (It is not unusual for a UART to also include some GPIO pins ...
  • 16.2k
1 vote

Does the 6522 VIA swap the registers for ports A and B compared to the 6820 PIA and 6526 CIA?

TL;DR The 6520/21 does not swap the ports, but differs from all other MOS interface chips, as Data and Direction registers share the same addresses. There is no flipping of sequence or flipping back ...
  • 178k

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