6522 VERSATILE INTERFACE ADAPTER This one is take from the VIC 20 Programmers Reference Guide which has got Commodores name all over it so I'm guessing it might be a bit more correct. There were a few mistakes that I picked up, so keep your eyes peeled because there could be more. ---------------------------------------------------------------------------- The 6522 Versatile Interface Adapter (VIA) provides the VIC with two peripheral ports with input latching, two powerful interval timers, and a serial-to-parallel/parallel-to-serial shift register. Basically, the VIC chip handles Audio/Video input/output, and the 6522 handles the rest. . . cassete operations, joysticks, RS-232, and user port. 6522 Versatile Interface Adapter Description ADDRESS DESCRIPTION REGISTER 9110 Port B AAAAAAAA 9111 Port A (with handshaking) BBBBBBBB 9112 Data Direction B CCCCCCCC 9113 Data Direction A DDDDDDDD 9114 Timer #1, low byte EEEEEEEE 9115 Timer #1, high byte FFFFFFFF 9116 Timer #1, low byte to load GGGGGGGG 9117 Timer #1, high byte to load HHHHHHHH 9118 Timer #2, low byte IIIIIIII 9119 Timer #2, high byte JJJJJJJJ 911A Shift Register KKKKKKKK 911B Auxiliary Control LLMNNNOP 911C Peripheral Control QQQRSSST 911D Interrupt Flags UVWXYZab 911E Interrupt Enable cdefghij 911F Port A (no handshaking) kkkkkkkk PORT B I/O REGISTER These eight bits are connected to the eight pins which make up port B. Each pin can be set for either input or output. Input latching is available on this port. When latch mode is enabled the data in the register freezes when the CB1 interrupt flag is set. The register stays latched until the interrupt flag is cleared. Handshaking is available for output from this port. CB2 will act as a DATA READY SIGNAL. This must be controlled by the user program. CB1 acts as the DATA ACCECPTED signal, and must be controlled by the device connected to the port. When DATA ACCEPTED is sent to the 6522, the DATA READY line is cleared, and the interrupt flag is set. PORT A I/O REGISTER These eight bits are connected to the eight pins which make up port A. Each pin can be set for either input or output. Handshaking is available for both read and write operations. Write handshaking is similar to that on PORT B. Read handshaking is automatic. The CA1 input pin acts as a DATA READY signal. The CA2 pin (used for output) is used for a DATA ACCEPTED signal. When a DATA READY signal is received a flag is set. The chip can be set to generate an interrupt or the flag can be polled under program control. The DATA ACCEPTED signal can either be a pulse or a DC level. It is set low by the CPU and cleared by the DATA READY signal. DATA DIRECTION FOR PORT B This register is used to control whether a particular bit in PORT B is used for input or output. Each bit of the data direction register (DDR) is associated with a bit of port B. If a bit in the DDR is set to 1, the corresponding bit of the port will be an OUTPUT. If a bit in the DDR is 0, the corresponding bit of the port will be an INPUT. For example, if the DDR is set to 7, port B will be set up as follows: BITS NUMBER DDR PORT B FUNCTION 0 1 OUTPUT 1 1 OUTPUT 2 1 OUTPUT 3 0 INPUT 4 0 INPUT 5 0 INPUT 6 0 INPUT 7 0 INPUT DATA DIRECTION REGISTER FOR PORT A This is similar to the DDR for port B, except that it works on PORT A. ------------------------------------------------------------------------------ TIMER CONTROLS There are two timers on the 6522 chip. The timers can be set to count down automatically or count pulses received by the VIA. The mode of operation is selected by the Auxiliary Control Register. TIMER T1 on the 6522 consists of two 8-bit latches and a 16-bit counter. The various modes of the TIMER are selected by setting the AUXILIARY CONTROL REGISTER (ACR). The latches are used to store a 16-bit data word to load into the counter. Loading a number into the latches does not affect the count in progress. After it is set, the counter will begin decrementing at 1 MHz. When the counter reaches zero, an interrupt flag will be set, and the IRQ will go low. Depending on how the TIMER is set, either further interrupts will be disabled, or it will automatically load the two latches into the counter and continue counting. The TIMER can also be set to invert the output signal on a peripheral pin each time it reaches zero and resets. The TIMER locations work differently on reading and writing. WRITING TO THE TIMER E: Write into the low order latch. This latch can be loaded into the low byte of the 16-bit counter. F: Write into the high order latch, write into the high order counter, transfer low order latch into the low order counter, and reset the TIMER T1 interrupt flag. In other words, when this location is set the counter is loaded. G: Same as E. H: Write into the high order latch and reset the TIMER T1 interrupt flag. READ THE TIMER E: Read the TIMER T1 low order counter and reset the TIMER T1 interrupt flag. F: Read the TIMER T1 high order counter. G: Read the TIMER T1 low order latch. H: Read the TIMER T1 high order latch. ------------------------------------------------------------------------------ TIMER T2 This TIMER operates as an interval timer (in one-shot mode), or as a counter for counting negative pulses on PORT B pin 6. A bit in the ACR selected which mode TIMER T2 is in. WRITING TO TIMER T2 I: Write TIMER T2 low order byte of latch. J: Write TIMER T2 high order counter byte, transfer low order latch to low order counter, clear TIMER T2 interrupt flag. READING TIMER T2 I: Read TIMER T2 low order counter byte, and clear TIMER T2 interrupt flag. J: Read TIMER T2 high order counter byte. ---------------------------------------------------------------------------- K: SHIFT REGISTER A shift register is a register which will rotate itself through the CB2 pin. The shift register can be loaded with any 8-bit pattern which can be shifted out through the CB1 pin, or input to the CB1 pin can be shifted into the shift register and then read. This makes it highly useful for serial to parallel and parallel to serial conversions. The shift register is controlled by bits 2-4 of the Auxiliary Control Register. ------------------------------------------------------------------------------ L,M,N,O,P: AUXILIARY CONTROL REGISTER ------------------------------------------------------------------------------ L: TIMER 1 CONTROL BIT # 7 6 0 0 One-shot mode (output to PB7 disabled) 0 1 Free running mode (output to PB7 disabled) 1 0 One-shot mode (output to PB7 enabled) 1 1 Free running mode (output to PB7 enabled) M: TIMER 2 CONTROL TIMER 2 has 2 modes. If this bit is 0, TIMER 2 acrs as an interval timer in one-shot mode. If this bit is 1, TIMER 2 will count a predetermined number on pin PB6. N: SHIFT REGISTER CONTROL BIT # 4 3 2 0 0 0 SHIFT REGISTER DISABLED 0 0 1 SHIFT IN (FROM CB1) UNDER CONTROL OF TIMER 2 0 1 0 SHIFT IN UNDER CONTROL OF SYSTEM CLOCK PULSES 0 1 1 SHIFT IN UNDER CONTROL OF EXTERNAL CLOCK PULSES 1 0 0 FREE RUN MODE AT RATE SET BY TIMER 2 1 0 1 SHIFT OUT UNDER CONTROL OF TIMER 2 1 1 0 SHIFT OUT UNDER CONTROL OF SYSTEM CLOCK PULSES 1 1 1 SHIFT OUT UNDER CONTROL OF EXTERNAL CLOCK PULSES O: PORT B LATCH ENABLE As long as this bit is 0, the PORT B register will directly reflect the data on the pins. If this bit is set to one, the data present on the input pins of PORT B will be latched within the chip when the CB1 INTERRUPT FLAG is set. As long as the CB1 INTERRUPT FLAG is set, the data on the pins can change without affecting the contents of the PORT B register. Note that the CPU always reads the register (the latches) rather than the pins. Input latching can be used with any of the input or output modes available for CB2. P: PORT A LATCH ENABLE As long as this bit is 0, the PORT A register will directly reflect the data on the pins. If this bit is set to one, the data present on the input pins of PORT A will be latched within the chip when the CA1 INTERRUPT FLAG is set. As long as the CA1 INTERRUPT FLAG is set, the data on the pins can change without affecting the contents of the PORT A register. Note that the CPU always reads the register (the latches) rather than the pins. Input latching can be used with any of the input or output modes available for CA2. ------------------------------------------------------------------------------ Q,R,S,T: THE PERIPHERAL CONTROL REGISTER ------------------------------------------------------------------------------ Q: CB2 CONTROL BIT # 7 6 5 0 0 0 Interrupt Input Mode 0 0 1 Independent Interrupt Input Mode 0 1 0 Input Mode 0 1 1 Independent Input Mode 1 0 0 Handshake Output Mode 1 0 1 Pulse Output Mode 1 1 0 Manual Output Mode (CB2 is held LOW) 1 1 1 Manual Output Mode (CB2 is held HIGH) INTERRUPT INPUT MODE: The CB2 interrupt flag (IFR bit 3) will be set on a negative (high-to-low) transition on the CB2 input line. The CB2 interrupt bit will be cleared on a read or write to PORT B. INDEPENDENT INTERRUPT INPUT MODE: As above, the CB2 interrupt flag will be set on a negative transition on the CB2 input line. However, reading or writing to PORT B does not clear the flag. INPUT MODE: The CB2 interrupt flag (IFR bit 3) will be set on a positive (low-to-high) transition of the CB2 line. The CB2 flag will be cleared on a read or write of PORT B. INDEPENDENT INPUT MODE: As above, the CB2 interrupt flag will be set on a positive transition on the CB2 line. However, reading or writing PORT B does not affect the flag. HANDSHAKE OUTPUT MODE: The CB2 line will be set low on a write to PORT B. It will be reset high again when there is an active transition on the CB1 line. PULSE OUTPUT MODE: The CB2 line is set low for one cycle after a write to PORT B. MANUAL OUTPUT MODE: The CB2 line is held low: MANUAL OUTPUT MODE: The CB2 line is held high. R: CB1 CONTROL This bit selects the active transition of the input signal applied to the CB1 pin. If this bit is 0, the CB1 interrupt flag will be set on a negative transistion (high-to-low). If this bit is a 1, the CB1 interrupt flag will be set on a positive (low-to-high) transition. S: CA2 CONTROL BIT # 3 2 1 0 0 0 Interrupt Input Mode 0 0 1 Independent Interrupt Input Mode 0 1 0 Input Mode 0 1 1 Independent Input Mode 1 0 0 Handshake Output Mode 1 0 1 Pulse Output Mode 1 1 0 Manual Output Mode (CA2 is held LOW) 1 1 1 Manual Output Mode (CA2 is held HIGH) INTERRUPT INPUT MODE: The CA2 interrupt flag (IFR bit 0) will be set on a negative (high-to-low) transition on the CA2 input line. The CA2 interrupt bit will be cleared on a read or write to PORT A. INDEPENDENT INTERRUPT INPUT MODE: As above, the CA2 interrupt flag will be set on a negative transition on the CA2 input line. However, reading or writing to PORT A does not clear the flag. INPUT MODE: The CA2 interrupt flag (IFR bit 0) will be set on a positive (low-to-high) transition of the CA2 line. The CB2 flag will be cleared on a read or write of PORT A. INDEPENDENT INPUT MODE: As above, the CA2 interrupt flag will be set on a positive transition on the CA2 line. However, reading or writing PORT A does not affect the flag. HANDSHAKE OUTPUT MODE: The CA2 line will be set low on a write to PORT A. It will be reset high again when there is an active transition on the CA1 line. PULSE OUTPUT MODE: The CA2 line is set low for one cycle after a read or write to PORT A. MANUAL OUTPUT MODE: The CA2 line is held low: MANUAL OUTPUT MODE: The CA2 line is held high. T: CA1 CONTROL This bit of the PCR selects the active transition of the input signal applied to the CA1 pin. If this bit is 0, the CA1 interrupt flag will be set on a negative transistion (high-to-low). If this bit is a 1, the CA1 interrupt flag will be set on a positive (low-to-high) transition. There are two registers associated with interrupts: The INTERRUPT FLAG REGISTER (IFR) and the INTERRUPT ENABLE REGISTER (IER). The IFR has eight bits, each one connected to a register in the 6522. Each bit in the IFR has an associated bit in the IER. The flag is set when a register wants to interrupt. However, no interrupt will take place unless the corresponding bit in the IER is set. UVWXYZab: INTERRUPT FLAG REGISTER When the flag is set, the pin associated with that flag is attempting to interrupt the 6502. Bit U is not a normal flag. It goes high if both the flag and the corresponding bit in the INTERRUPT ENABLE REGISTER are set. It can be cleared only by clearing all the flags in the IFR or disabling all active interrupts in the IER. SET BY CLEARED BY U IRQ STATUS V TIMER 1 time-out Reading TIMER 1 low order counter and writing TIMER 1 high order latch. W TIMER 2 time-out Reading TIMER 2 low order counter and writing TIMER 2 high order counter. X CB1 pin active transition Reading or writing PORT B. Y CB2 pin active transition Reading or writing PORT B. Z Completion of 8 shifts Reading or writing the shift register. a CA1 pin active transition Reading or writing PORT A (BBBBBBBB). b CA2 pin active transition Reading or writing PORT A (BBBBBBBB). cdefghij: INTERRUPT ENABLE REGISTER c: ENABLE CONTROL If this bit is a 0 during a write to this register, each 1 in bits 0-6 clears the corresponding bit in the IER. If this bit is a 1 during a write to this register, each 1 in bits 0-6 will set the corresponding IER bit. d TIMER 1 time-out enable. e TIMER 2 time-out enable. f CB1 interrupt enable. g CB2 interrupt enable. h Shift interrupt enable. i CA1 interrupt enable. j CA2 interrupt enable. kkkkkkkk: PORT A This is similar to BBBBBBBB except that the handshaking lines (CA1 and CA2) are unaffected by operations on this port.