Rabbit Rabbit 4000, User Manual

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Chapter 18 Slave Port 167
18.3.7 Other Configurations
There are other slave port configurations possible:
•
The master could use the auxiliary I/O bus instead of the memory bus.
•
All devices could poll the slave port status register to determine when data is present
instead of relying on interrupts.
•
The master could write to SPD0R, triggering an interrupt on the slave. The slave could
then simply write a response into SPD0R, which the master detects by polling SPSR.
This configuration is useful when fewer signals are desired, or the master device has no
external interrupts available.
If polling is to be used, it is important to note that not all bits in the status register may be
updated at once; it is possible to read a transitional value as the register updates. To guar-
antee a proper polling read, the status register should be read twice; when the same value
is read both times the value is correct.
Similarly, it is possible to receive a scrambled value from a data register if it is read while
being written. The protocol used should take account of this and prevent it from occurring
(the protocol described above guarantees this will not occur).