Digi Rabbit 5000, User Manual

Page: 320 / 414

320 Rabbit 5000 Microprocessor User’s Manual
27.3 Operation
27.3.1 External I/O Bus
The following steps must be taken before using external I/O bus:
1. Enable the external I/O bus by writing to SPCR. Select whether 6 or 8 address bits are
desired.
2. If PB0 and PB1 are needed for clocked serial use and eight address bits are required,
enable the alternate outputs of address bits 6 and 7 on Parallel Port D by writing to
PDALR, PDAHR, and PDFR.
3. Set the I/O timing for a particular device by writing to the appropriate IB x CR register
for the I/O bank desired.
4. If a strobe other than /IORD, /IOWR, or /BUFEN is required, enable the output of the
IB x CR register by writing to the appropriate P x ALR, P x AHR, and P x FR registers.
Once the external I/O bus is enabled, all memory read/write instructions prefixed with an
IOE will go to either the memory bus or external I/O bus, depending on the setup in that
bank’s IB
x CR register.
27.3.2 I/O Strobes
The following steps must be taken before using an I/O strobe:
1. Set the strobe type and timing for a particular device by writing to the appropriate
IB x CR register for the I/O bank desired.
2. If signals other than /IORD, /IOWR, and /BUFEN are required, enable the output of the
IB x CR register by writing to the appropriate P x ALR, P x AHR, and P x FR registers.
On startup, the I/O strobes are set as chip selects with 15 wait states, read-only, active-low
signaling, and will use the external I/O bus. These settings will be used for the dedicated
I/O strobe pins /IORD, /IOWR, and /BUFEN whenever an external I/O write occurs even
if not I/O strobe signals are being output on parallel port pins.
27.3.3 I/O Handshake
The following steps must be taken before using the I/O handshake:
1. Select the active level and desired port E bit to use as input by writing to IHCR.
2. Select which I/O banks the handshake is active for by writing to IHSR.
3. Select the handshake timeout value by writing to IHTR.
Once enabled, the handshake will be checked for every external I/O transaction in a bank
that was enabled in IHSR. After these transactions, the program should check for a time-
out by reading IHTR.