FullDuplexSerial.spin and locks

Bob Belleville

FullDuplexSerial seems to work just fine.

It uses no locks even though the FiFos are multiple bytes.

Is there some aspect of the code I am missing the makes
the need to obtain a lock before reading or writing a FiFo
unnecessary?

many thanks -- Bob

Paul Baker

Locks are only needed if there are multiple producers or multiple consumers. In a standard single producer/single consumer, there is no conflict because each modifies the opposite end of the fifo and there is never a situation where data loss or multiplication can occur.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Mike Green

There is a special case of multi-processor access to shared variables known as the single producer / single consumer that doesn't need any locking.· Basically, only one processor can change the variable, but a second processor can read it.· This works nicely with a buffer in that there's an input pointer that's modified only by the processor writing into the buffer, but accessed by the processor reading the buffer (to see if there's anything there).· Similarly, there's an output pointer that's modified only by the processor reading the buffer, but accessed by the processor writing into the buffer (to see if there's room left).· If the writer thinks that the buffer is full even though the reader is in the process of removing a character, there's no problem since the writer will wait.· Similarly, if the reader thinks that the buffer is empty even if the writer is putting a new character into the buffer, there's no problem since the reader will wait.
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Chris Savage

Another case where you would need to use locks or flags is if there is an array or group of data that needs to be updated before it can be read. In this case you don’t want one processor reading bytes from the array before they’re all written. Take care.

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Chris Savage
Parallax Tech Support