55.4.3 Processing Period

When SHA is enabled and DMA is used to write the messages, the inherent processing period may result, depending on the application, in a significant bandwidth usage at system bus level. In some applications, it may be important to keep as much bandwidth as possible for the other peripherals (e.g. CPU, other DMA channels). The SHA engine inherent processing period can be configured to reduce the bandwidth required by writing SHA_MR.PROCDLY=1.

In SHA1 mode, the shortest processing period is 85 clock cycles + 2 clock cycles for start command synchronization (SHA_MR.PROCDLY=0). The longest period is 209 clock cycles + 2 clock cycles when SHA_MR.PROCDLY=1 (see the figure below).

In SHA256 mode, the shortest processing period is 72 clock cycles + 2 clock cycles for start command synchronization (SHA_MR.PROCDLY=0). The longest period is 194 clock cycles + 2 clock cycles when SHA_MR.PROCDLY=1.

In SHA384 or SHA512 mode, the shortest processing period is 88 clock cycles + 2 clock cycles for start command synchronization. The longest period is 209 clock cycles + 2 clock cycles.