There are two 65C22 "Versatile Interface Adapter" (VIA) I/O controllers in the system, VIA#1 at address $9F00 and VIA#2 at address $9F10. The IRQ out lines of VIA#1 is connected to the CPU's NMI line, while the IRQ out line of VIA#2 is connected to the CPU's IRQ line.
The-following tables describe the connections of the I/O pins:
VIA#1
| Pin | Name | Description |
|---|---|---|
| PA0 | I2CDATA | I2C Data |
| PA1 | I2CCLK | I2C Clock |
| PA2 | NESLATCH | NES LATCH (for all controllers) |
| PA3 | NESCLK | NES CLK (for all controllers) |
| PA4 | NESDAT3 | NES DATA (controller 3) |
| PA5 | NESDAT2 | NES DATA (controller 2) |
| PA6 | NESDAT1 | NES DATA (controller 1) |
| PA7 | NESDAT0 | NES DATA (controller 0) |
| PB0 | Unused | |
| PB1 | Unused | |
| PB2 | Unused | |
| PB3 | SERATNO | Serial ATN out |
| PB4 | SERCLKO | Serial CLK out |
| PB5 | SERDATAO | Serial DATA out |
| PB6 | SERCLKI | Serial CLK in |
| PB7 | SERDATAI | Serial DATA in |
| CA1 | Unused | |
| CA2 | Unused | |
| CB1 | IECSRQ | |
| CB2 | Unused |
The KERNAL uses Timer 2 for timing transmissions on the Serial Bus.
VIA#2
The second VIA is completely unused by the system. All its 16 GPIOs and 4 handshake I/Os can be freely used.
The Commander X16 contains an I2C bus, which is implemented through two pins of VIA#1. The system management controller (SMC) and the real-time clock (RTC) are connected through this bus. The KERNAL APIs i2c_read_byte and i2c_write_byte allow talking to these devices.
The system management controller (SMC) is device $42 on the I2C bus. It controls the power and activity LEDs, and can be used to power down the system or inject RESET and NMI signals.
| Register | Value | Description |
|---|---|---|
| $01 | $00 | Power off |
| $01 | $01 | Hard reboot |
| $02 | $00 | Inject RESET |
| $03 | $00 | Inject NMI |
| $04 | $00..$FF | Power LED brightness |
| $05 | $00..$FF | Activity LED brightness |
| $07 | - | Read from keyboard buffer |
| $18 | - | Read ps2 status |
| $19 | $00..$FF | Send ps2 command |
| $21 | - | Read from mouse buffer |
The Commander X16 contains a battery-backed Microchip MCP7940N real-time-clock (RTC) chip as device $6F. It provide a real-time clock/calendar, two alarms and 64 bytes of RAM.
| Register | Description |
|---|---|
| $00 | Clock seconds |
| $01 | Clock minutes |
| $02 | Clock hours |
| $03 | Clock weekday |
| $04 | Clock day |
| $05 | Clock month |
| $06 | Clock year |
| $07 | Control |
| $08 | Oscillator trim |
| $09 | reserved |
| $0A | Alarm 0 seconds |
| $0B | Alarm 0 minutes |
| $0C | Alarm 0 hours |
| $0D | Alarm 0 weekday |
| $0E | Alarm 0 day |
| $0F | Alarm 0 month |
| $10 | reserved |
| $11 | Alarm 1 seconds |
| $12 | Alarm 1 minutes |
| $13 | Alarm 1 hours |
| $14 | Alarm 1 weekday |
| $15 | Alarm 1 day |
| $16 | Alarm 1 month |
| $17 | reserved |
| $18 | Power-fail minutes |
| $19 | Power-fail hours |
| $1A | Power-fail day |
| $1B | Power-fail month |
| $1C | Power-up minutes |
| $1D | Power-up hours |
| $1E | Power-up day |
| $1F | Power-up month |
| $20-$5F | 64 Bytes SRAM |
The second half of the RTC's SRAM (NVRAM) is reserved for use by the KERNAL. $20-$3F is available for use by user programs.
For more information, please refer to this device's datasheet.