CIA

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

Description[edit]

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[edit]

  • 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 adresses of the CIAs[edit]

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

CIA 1[edit]

Paddles,
Adress area: $DC00-$DCFF, 56320-56575 Tasks: Keyboard, Joystick, Paddles, Datasette, IRQ control
Adress
Hex
Adress
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[edit]

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.

Adress range: $DD00-$DDFF, 56576-56831 Tasks: Serial bus, RS-232, VIC memory, NMI control
Adress
Hex
Adress
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[edit]

Pin Description
CIA 6526 - Pinbelegung.png 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[edit]

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[edit]

WP-W11.png Wikipedia: MOS_Technology_CIA

Hardware Assembly 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