Zilog Z08470 - Manual (Page 295)

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 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)