Z80 Datasheet, PDF (295/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

Z80 CPU Peripherals
User Manual
275
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
0
1
1 Channel Reset
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)
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.
UM008101-0601
Serial Input/Output