CIA

Production year/date of this CIA: 1985 - calendar week 13

Description

The CIA (Complex Interface Adapter) is an interface chip used in the Commodore homecomputer. It controls most of the I/O processes and contains as well the internal timer (clock). The CIA was developed from the Semiconductor manufacturer MOS Technology, which was taken over in 1976 by Commodore. Inside the C64 there are two CIA 6526 chips used; later C64-versions may also use the 6526A or the 8521.

Details of the CIA 6526

• 16 single programmable In- and Output lines
  • The lines are lead through open collector with internal pullups.
  • In the C64 some of these lines are used to monitor/control the I/O devices (keyboard, joystick, iec, etc.), some are connected with the VIC to define which area of memory it can address, others are for free disposition and are available at the userport.
• 8- or 16-Bit data transport (reading or writing) with handshaking.
• 2 independent 16-bit interval timers
  • each timer consists of a 16 bit-latch (start value) and the actual 16 bit-timer
  • the latch can be set directly, the timer can only be read indirectly via the latch.
• 24-h-timeclock (AM/PM) with programmable alarm
• 8-bit shift register for serial In- and Output.
• 2 TTL-inputs
• CMOS-compatible
• Pulsing: 1 MHz (6526) or 2 MHz (6526A)

Memory addresses of the CIAs

The following memory addresses are used from the two CIA-Chips in the C64:

CIA 1

Paddles,

Address area: $DC00-$DCFF, 56320-56575 Tasks: Keyboard, Joystick, Paddles, Datasette, IRQ control
Address Hex	Address Dec	Register	Function	Remark
$DC00	56320	0 PRA	Data Port A	Monitoring/control of the 8 data lines of Port A. The lines are used for multiple purposes: Read/Write: Bit 0..7 keyboard matrix columns Read: Joystick Port 2: Bit 0..3 Direction (Left/Right/Up/Down), Bit 4 Fire button. 0 = activated. Read: Lightpen: Bit 4 (as fire button), connected also with "/LP" (Pin 9) of the VIC Read: Paddles: Bit 2..3 Fire buttons, Bit 6..7 Switch control port 1 (%01=Paddles A) or 2 (%10=Paddles B)
$DC01	56321	1 PRB	Data Port B	Monitoring/control of the 8 data lines of Port B. The lines are used for multiple purposes: Read/Write: Bit 0..7 keyboard matrix rows Read: Joystick Port 1: Bit 0..3 Direction (Left/Right/Up/Down), Bit 4 Fire button. 0 = activated. Read: Bit 6: Timer A: Toggle/Impulse output (see register 14 bit 2) Read: Bit 7: Timer B: Toggle/Impulse output (see register 15 bit 2)
$DC02	56322	2 DDRA	Data Direction Port A	Bit X: 0=Input (read only), 1=Output (read and write)
$DC03	56323	3 DDRB	Data Direction Port B	Bit X: 0=Input (read only), 1=Output (read and write)
$DC04	56324	4 TA LO	Timer A Low Byte	Read: actual value Timer A (Low Byte) Writing: Set latch of Timer A (Low Byte)
$DC05	56325	5 TA HI	Timer A High Byte	Read: actual value Timer A (High Byte) Writing: Set latch of timer A (High Byte) - if the timer is stopped, the high-byte will automatically be re-set as well
$DC06	56326	6 TB LO	Timer B Low Byte	Read: actual value Timer B (Low Byte) Writing: Set latch of Timer B (Low Byte)
$DC07	56327	7 TB HI	Timer B High Byte	Read: actual value Timer B (High Byte) Writing: Set latch of timer B (High Byte) - if the timer is stopped, the high-byte will automatically be re-set as well
$DC08	56328	8 TOD 10THS	Real Time Clock 1/10s	Read: Bit 0..3: Tenth seconds in BCD-format ($0-$9) Bit 4..7: always 0 Writing: Bit 0..3: if CRB-Bit7=0: Set the tenth seconds in BCD-format Bit 0..3: if CRB-Bit7=1: Set the tenth seconds of the alarm time in BCD-format
$DC09	56329	9 TOD SEC	Real Time Clock Seconds	Bit 0..3: Single seconds in BCD-format ($0-$9) Bit 4..6: Ten seconds in BCD-format ($0-$5) Bit 7: always 0
$DC0A	56330	10 TOD MIN	Real Time Clock Minutes	Bit 0..3: Single minutes in BCD-format( $0-$9) Bit 4..6: Ten minutes in BCD-format ($0-$5) Bit 7: always 0
$DC0B	56331	11 TOD HR	Real Time Clock Hours	Bit 0..3: Single hours in BCD-format ($0-$9) Bit 4..6: Ten hours in BCD-format ($0-$5) Bit 7: Differentiation AM/PM, 0=AM, 1=PM Writing into this register stops TOD, until register 8 (TOD 10THS) will be read.
$DC0C	56332	12 SDR	Serial shift register	The byte within this register will be shifted bitwise to or from the SP-pin with every positive slope at the CNT-pin.
$DC0D	56333	13 ICR	Interrupt Control and status	CIA1 is connected to the IRQ-Line. Read: (Bit0..4 = INT DATA, Origin of the interrupt) Bit 0: 1 = Underflow Timer A Bit 1: 1 = Underflow Timer B Bit 2: 1 = Time of day and alarm time is equal Bit 3: 1 = SDR full or empty, so full byte was transferred, depending of operating mode serial bus Bit 4: 1 = IRQ Signal occured at FLAG-pin (cassette port Data input, serial bus SRQ IN) Bit 5..6: always 0 Bit 7: 1 = IRQ An interrupt occured, so at least one bit of INT MASK and INT DATA is set in both registers. Flags will be cleared after reading the register! Write: (Bit 0..4 = INT MASK, Interrupt mask) Bit 0: 1 = Interrupt release through timer A underflow Bit 1: 1 = Interrupt release through timer B underflow Bit 2: 1 = Interrupt release if clock=alarmtime Bit 3: 1 = Interrupt release if a complete byte has been received/sent. Bit 4: 1 = Interrupt release if a positive slope occurs at the FLAG-Pin. Bit 5..6: unused Bit 7: Source bit. 0 = set bits 0..4 are clearing the according mask bit. 1 = set bits 0..4 are setting the according mask bit. If all bits 0..4 are cleared, there will be no change to the mask.
$DC0E	56334	14 CRA	Control Timer A	Bit 0: 0 = Stop timer; 1 = Start timer Bit 1: 1 = Indicates a timer underflow at port B in bit 6. Bit 2: 0 = Through a timer overflow, bit 6 of port B will get high for one cycle , 1 = Through a timer underflow, bit 6 of port B will be inverted Bit 3: 0 = Timer-restart after underflow (latch will be reloaded), 1 = Timer stops after underflow. Bit 4: 1 = Load latch into the timer once. Bit 5: 0 = Timer counts system cycles, 1 = Timer counts positive slope at CNT-pin Bit 6: Direction of the serial shift register, 0 = SP-pin is input (read), 1 = SP-pin is output (write) Bit 7: Real Time Clock, 0 = 60 Hz, 1 = 50 Hz
$DC0F	56335	15 CRB	Control Timer B	Bit 0: 0 = Stop timer; 1 = Start timer Bit 1: 1 = Indicates a timer underflow at port B in bit 7. Bit 2: 0 = Through a timer overflow, bit 7 of port B will get high for one cycle , 1 = Through a timer underflow, bit 7 of port B will be inverted Bit 3: 0 = Timer-restart after underflow (latch will be reloaded), 1 = Timer stops after underflow. Bit 4: 1 = Load latch into the timer once. Bit 5..6: %00 = Timer counts System cycle %01 = Timer counts positive slope on CNT-pin %10 = Timer counts underflow of timer A %11 = Timer counts underflow of timer A if the CNT-pin is high Bit 7: 0 = Writing into the TOD register sets the clock time, 1 = Writing into the TOD register sets the alarm time.
$DC10-$DCFF	56336-56575	-	-	The CIA 1 register are mirrored each 16 Bytes

CIA 2

The second CIA-chip is identical to the first. Therefore in the following table are only entries which are specific to the usage in the C64.

Address range: $DD00-$DDFF, 56576-56831 Tasks: Serial bus, RS-232, VIC memory, NMI control
Address Hex	Address Dec	Register	Function	Remark
$DD00	56576	0 PRA	Data Port A	Bit 0..1: Select the position of the VIC-memory %00, 0: Bank 3: $C000-$FFFF, 49152-65535 %01, 1: Bank 2: $8000-$BFFF, 32768-49151 %10, 2: Bank 1: $4000-$7FFF, 16384-32767 %11, 3: Bank 0: $0000-$3FFF, 0-16383 (standard) Bit 2: RS-232: TXD Output, userport: Data PA 2 (pin M) Bit 3..5: serial bus Output (0=High/Inactive, 1=Low/Active) Bit 3: ATN OUT Bit 4: CLOCK OUT Bit 5: DATA OUT Bit 6..7: serial bus Input (0=Low/Active, 1=High/Inactive) Bit 6: CLOCK IN Bit 7: DATA IN
$DD01	56577	1 PRB	Data Port B	Bit 0..7: userport Data PB 0-7 (Pins C,D,E,F,H,J,K,L) The KERNAL offers several RS232-Routines, which use the pins as followed: Bit 0, 3..7: RS-232: reading Bit 0: RXD Bit 3: RI Bit 4: DCD Bit 5: User port pin J Bit 6: CTS Bit 7: DSR Bit 1..5: RS-232: writing Bit 1: RTS Bit 2: DTR Bit 3: RI Bit 4: DCD Bit 5: User port pin J
$DD02	56578	2 DDRA	Data direction Port A	see CIA 1
$DD03	56579	3 DDRB	Data direction Port B	see CIA 1
$DD04	56580	4 TA LO	Timer A Low Byte	see CIA 1
$DD05	56581	5 TA HI	Timer A High Byte	see CIA 1
$DD06	56582	6 TB LO	Timer B Low Byte	see CIA 1
$DD07	56583	7 TB HI	Timer B High Byte	see CIA 1
$DD08	56584	8 TOD 10THS	Real Time Clock 1/10s	see CIA 1
$DD09	56585	9 TOD SEC	Real Time Clock Seconds	see CIA 1
$DD0A	56586	10 TOD MIN	Real Time Clock Minutes	see CIA 1
$DD0B	56587	11 TOD HR	Real Time Clock Hours	see CIA 1
$DD0C	56588	12 SDR	Serial shift register	see CIA 1
$DD0D	56589	13 ICR	Interrupt control and status	CIA2 is connected to the NMI-Line. Bit 4: 1 = NMI Signal occured at FLAG-pin (RS-232 data received) Bit 7: 1 = NMI An interrupt occured, so at least one bit of INT MASK and INT DATA is set in both registers.
$DD0E	56590	14 CRA	Control Timer A	see CIA 1
$DD0F	56591	15 CRB	Control Timer B	see CIA 1

Pinout

	Pin	Description
	Vss	Ground connection 0V
	PA0 - PA7	I/O-Port A Bit0-7 Bidirectional parallell port
	PB0 - PB7	I/O-Port B Bit0-7 Bidirectional parallell port
	/PC	Port Control - Handshake output, Low pulse after read/write on port B.
	TOD	Time Of Day clock input, either 50Hz or 60Hz input.
	Vcc	5VDC input
	/IRQ	Interrupt Request, output to CPU's IRQ input
	R/W	High-level = Read (Move data from CIA), Low-level = Write (Move data to CIA)
	/CS	Chip Select - low level means active CIA.
	/FLAG	Negative edge IRQ input, can be used as handshake for either parallel port.
	/phi2	Processor Φ2 clock, data transfers occur only on high signal level.
	DB0 - DB7	Bidirectional data bus Bits 0 - 7
	/RES	Reset input, low signal initiates CIA.
	RS0 - RS3	Register Select - Used to interface internal registers with bidirectional ports.
	SP	Serial Port - bidirectional, internal shift register converts CPU parallel data into serial data and vice-versa.
	CNT	Count - Internal timers can count pulses to this input. Can be used for frequency dependent operations.

Failure Symptoms

CIA 1

• Startup screen normal, but no cursor.
• No keyboard or control port access but cartridge works.
• Partial failure: some keys or joystick positions don't work, one character appears ahead of startup cursor or random characters appear at startup. Blank screen if chip is shorted (remove to check) and chip may get hot to the touch.

CIA 2

• Startup screen normal.
• No serial or user port access.
• "File not found" error when drive accessed.
• Cartridge works.
• Characters may show as blocks on startup screen.

Links

Wikipedia: MOS_Technology_CIA

• MOS 6526 CIA data sheet (PDF Format)
• Mapping the C64 - e.g. details about programming the CIA
• The CIA 6526 in the Commodore Computer Online Museum

Hardware internals of the C64 · Hardware portal
Components of the C64: CPU · VIC · SID · CIA · PLA · RAM · ROM · Color RAM

Interfaces of the C64: Control Port · Expansion Port · Userport · Cassette Port · Power Supply Connector · A/V Jack · RF Jack · Serial Port