MOS Technology CIA
The MOS Technology CIA ( Complex Interface Adapter ) is a versatile I / O - chip , which in many Commodore Computer? And peripherals was used (including C64 and Amiga ). It was manufactured by MOS Technology and is the successor to the MOS Technology VIA .
There are four types of the CIA: 6526 , 8521 and 8520 , the latter being used in the Amigas and in the VC1581 floppy drive . An integrated version 5710 with only three registers was developed for the C128DCR .
The CIAs 6526 and 8521 have two bidirectional 8-bit TTL input / output ports (all bits can be programmed independently for input or output), a serial shift register , 2 cascadable 16-bit timers and a real-time clock accurate to a tenth of a second with alarm function. The 8520 has a 24-bit timers instead of the RTC. If the timer underflows, the shift register is full or empty, the alarm time is reached or an external signal is received, the chip can trigger an interrupt if required .
Each data line of the input / output ports can be switched individually to input or output.
The real-time clock uses the 50 or 60 Hz mains frequency as a reference, the time can be read out via four registers (tenths of a second, seconds, minutes, hours) in BCD format. In addition, an alarm time can be set at which an interrupt is triggered.
The timers can be triggered by an internal clock, but they can also count external signals. A counter underflow can trigger an interrupt and / or send a pulse to an input / output pin.