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Z80 CPU Peripherals

User Manual

UM008101-0601 Serial Input/Output

275

Command 0 (Null). The Null command has no effect. Normally the null

command instructs the Z80 SIO to wait while the pointers are set for the

following byte.

Command 1 (Send Abort). This command is used only in SDLC mode to

generate a sequence of eight to thirteen 1s.

Command 2 (Reset External/Status Interrupts). After an External/Status

interrupt, such as a change on a modem line or a break condition, the status

bits of RR0 are latched. This command re-enables them, again allowing

Interrupts to occur. Latching the status bits captures short pulses until the

CPU has time to read the change.

Command 3 (Channel Reset). This command performs the same function

as an External Reset, but on a single channel. Channel A Reset also resets

the interrupt prioritization logic. All control registers for the channel must

be rewritten after a Channel Reset command.

After a Z80 SIO Channel Reset, add four system clock cycles before addi-

tional commands or controls write to that channel. This is normally the

same amount of time that is used by the CPU to fetch the next Op Code.

Table 13. Z80 SIO Commands

Command CMD2 CMD1 CMD0 Result

0 0 0 0 Null Command (no effect)

1 0 0 1 Send Abort (SDLC Mode)

2 0 1 0 Reset External/Status Interrupts

3 011ChannelReset

4 1 0 0 Enable Interrupt on next Rx Character

5 1 0 1 Reset Transmitter Interrupt Pending

6 1 1 0 Error Reset (latches

7 1 1 1 Return from Interrupt (Channel A)

1 2 ... 290 291 292 293 294 295 296 297 298 299 300 ... 329 330

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