The CIA (Complex Interface Adapter) is a peripheral IC which is used in the C-64 for controlling the serial bus and the keyboard, among other things.
Technical Data
![[Picture of 6526]](pics/6526.gif)
| Pin(s) | Signal | Dir | Description |
|---|---|---|---|
| 1 | GND | - | Ground (0V). |
| 2-9 | PA0-PA7 | progr. | Port A 0-7. |
| 10-17 | PB0-PB7 | progr. | Port B 0-7. With the control registers A and B, bits 6 and 7 can be programmed to indicate an underflow of timer A and B. |
| 18 | /PC | out | Port control. Indicates availability of data on port B or both ports [1]. |
| 19 | TOD | in | Time of day. A TTL signal carrying the mains frequency (derived from the 9VAC) of 50Hz (PAL) / 60Hz (NTSC) is applied here to trigger the realtime clock. |
| 20 | Vcc | - | Supply voltage (+5V DC). |
| 21 | /IRQ | out | Interrupt ReQuest. Becomes LOW when it matches a set bit in the interrupt control register on ocurrence of the corresponding event. In the C64, this line is connected to the CPU's /IRQ pin (CIA #1) or to the /NMI pin (CIA #2), respectively. |
| 22 | R/-W | in | Read/-Write. 0=read on data bus, 1=write on data bus. |
| 23 | /CS | in | Chip select. 0=coupled to data bus, 1=tri-state. |
| 24 | /FLAG | in | Flag input. See /PC (pin 18). |
| 25 | ø 2* | in | Phi 2. System clock signal. All data bus action takes place only when ø2=1. |
| 26-33 | DB7-DB0 | in/out | Data bus. |
| 34 | /RES | in | Reset. 0=reset the CIA to initial state. |
| 35-38 | RS3-RS0 | in | Register select. These four pins select one of the CIA's internal registers. |
| 39 | SP | in/out | Serial port. In/output of the shift register. |
| 40 | CNT | in/out(?) | Counter. In/(out??)put of shift register clock or trigger input for interval timer [1]. |
In the 1570/71/81, the CIA is used to implement the fast serial bus.
Wolfgang Lorenz has detailled information in his article 'A Software Model of the CIA6526'. This article, a test suite for testing CIA compatibility of emulators, and other files belonging to it can be found on Funet.