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Chapter 27 External I/O Control
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27.2 Dependencies
27.2.1 I/O Pins
The external I/O bus uses PA0–PA7 for data, and either PB2–PB7 or PB0–PB7 for address
lines, depending on the setting in SPCR. Address bits 6 and 7 can also be enabled on pins
PD1, PD3, PD5, or PD7, which allows PB0 and PB1 to be used as clocked serial I/O
instead of as external I/O.
The /IOWR, /IORD, and /BUFEN pins are dedicated strobes for external I/O accesses.
The I/O strobes can be directed out to pins on Parallel Ports C, D, or E; each bank can be
directed to the appropriate pin (bank zero on PC0, PD0, or PE0; bank one on PC1, PD1, or
PE1; etc.). The strobes will affect outputs on /IOWR, /IORD, and /BUFEN at all times.
The I/O handshake can be input on any one of the Parallel Port E pins (PE0–PE7).
27.2.2 Clocks
All external I/O accesses, strobes, and handshake timeouts are based on the processor
clock.
27.2.3 Other Registers
27.2.4 Interrupts
There are no interrupts associated with external I/O.
Register
Function
SPCR
Enable the external I/O bus.
PCFR, PCALR, PCAHR
PDFR, PDALR, PDAHR,
PEFR, PEALR, PEAHR
Select Parallel Port C, D, or E pins as I/O strobe
outputs.
Select PD1, PD3, PD5, or PD7 as address bits 6-7.
Summary of Contents for Rabbit 5000
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