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Rabbit 6000 User’s Manual
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354
31.3 Operation
31.3.1 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. Select 8- or 16-bit mode and set the I/O timing for a particular device by writing to the appropriate
IB
x
CR and IBxER registers for the I/O bank desired. Make sure to set bit 3 in IB
x
CR if write access
is desired.
4. If a strobe other than /IORD, /IOWR, or /BUFEN is required, enable the output of the IB
x
CR regis-
ter 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
the memory bus and/or external I/O bus, depending on the setup in that bank’s IB
x
CR register.
31.3.2 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 and
IBxER registers 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 reg-
ister by writing to the appropriate P
x
ALR, P
x
AHR, and P
x
FR registers.
On startup, the I/O strobes default to 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 no I/O strobe signals are being output on par-
allel port pins.
31.3.3 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 timeout by reading IHTR.
Summary of Contents for 6000
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