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Z80 CPU Peripherals

User Manual

UM008101-0601 Serial Input/Output

211

When CE and IORQ are active, but RD is inactive, the channel selected by

B/A is written to by the CPU with either data or control information as

specified by C/D. As mentioned previously, if IORQ and M1 are active

simultaneously, the CPU is acknowledging an interrupt and the Z80 SIO

automatically places its interrupt vector on the CPU data bus if it is the

highest priority device requesting an interrupt.

Read Cycle Status (input from CPU, active Low). If RD is active, a

memory or I/O read operation is in progress. RD is used with B/A,CE,

and IORQ

to transfer data from the Z80 SIO to the CPU.

RESET

Reset (input, active Low). A Low /RESET disables both /RESET

and transmitters, forces TxDA and TxDB marking, forces the modem

controls High and disables all interrupts. The control registers must be

rewritten after the Z80 SIO is reset and before data is transmitted or received.

IEI Interrupt Enable In (input, active High). This signal is used with IEO

to form a priority daisy-chain when there is more than one interrupt-

driven device. A High on this line indicates that no other device of higher

priority is being serviced by a CPU interrupt service routine.

IEO Interrupt Enable Out (output, active High). IEO is High only if IEI is

High and the CPU is not servicing an interrupt from this Z80 SIO. There-

fore, this signal blocks lower priority devices from interrupting while a

higher priority device is being serviced by its CPU interrupt service routine.

INT

Interrupt Request (output, open-drain, active Low). When the Z80

SIO is requesting an interrupt, it pulls INT

Low.

W/RDYA

,W/RDYBWait/Ready A, Wait/Ready B (outputs, open-drain

when programmed for Wait function, driven High and Low when

programmed for Ready function). These dual-purpose outputs may be

programmed as Ready lines for a DMA controller or as Wait lines that

synchronize the CPU to the Z80 SIO data rate. The reset state is open-drain.

CSTA

,CSTBClear To Send (inputs, active Low). When programmed as

Auto Enables, a Low on these inputs enables the respective transmitter. If

not programmed as Auto Enables, these inputs may be programmed as

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