Rabbit 6000 User’s Manual
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222
21.3.7 Other Configurations
There are other slave port configurations possible:
•
The master could use the external 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 guarantee a proper polling read, the sta-
tus 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).
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