Context Swapping - STMicroelectronics STM32F405 Reference Manual

Cryptographic processor (CRYP)
the DES/TDES. The DMA should be configured to set an interrupt on transfer
completion of the output data to indicate that the processing is finished.
2. Enable the cryptographic processor by writing the CRYPEN bit to 1. Enable the DMA
requests by setting the DIEN and DOEN bits in the CRYP_DMACR register.
3. All the transfers and processing are managed by the DMA and the cryptographic
processor. The DMA interrupt indicates that the processing is complete. Both FIFOs
are normally empty and BUSY = 0.
Processing when the data are transferred by the CPU during interrupts
1. Enable the interrupts by setting the INIM and OUTIM bits in the CRYP_IMSCR register.
2. Enable the cryptographic processor by setting the CRYPEN bit in the CRYP_CR
register.
3. In the interrupt managing the input data: load the input message into the IN FIFO. You
can load 2 or 4 words at a time, or load data until the FIFO is full. When the last word of
the message has been entered into the FIFO, disable the interrupt by clearing the INIM
bit.
4. In the interrupt managing the output data: read the output message from the OUT
FIFO. You can read 1 block (2 or 4 words) at a time or read data until the FIFO is
empty. When the last word has been read, INIM=0, BUSY=0 and both FIFOs are empty
(IFEM=1 and OFNE=0). You can disable the interrupt by clearing the OUTIM bit and,
the peripheral by clearing the CRYPEN bit.
Processing without using the DMA nor interrupts
1. Enable the cryptographic processor by setting the CRYPEN bit in the CRYP_CR
register.
2. Write the first blocks in the input FIFO (2 to 8 words).
3. Repeat the following sequence until the complete message has been processed:
a)
b)
4. At the end of the processing, BUSY=0 and both FIFOs are empty (IFEM=1 and
OFNE=0). You can disable the peripheral by clearing the CRYPEN bit.
23.3.7

Context swapping

If a context switching is needed because a new task launched by the OS requires this
resource, the following tasks have to be performed for full context restoration (example
when the DMA is used):
736/1731
Wait for OFNE=1, then read the OUT-FIFO (1 block or until the FIFO is empty)
Wait for IFNF=1, then write the IN FIFO (1 block or until the FIFO is full)
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