10.7 Predictive Prefetch Behavior

When the user configures the module for predictive prefetch, the prefetch module predicts the next line address, fetches the instruction and, then, stores it in the prefetch buffer. If the requested instruction is not in a prefetch module line and the read address matches the predicted address, the content of the prefetch buffer is loaded in the prefetch module line while simultaneously returning the critical word to the read initiator.

On enabling the predictive prefetch, the prefetch function starts predicting based on the first address read to the PFM. When the user places the first line in the prefetch module, the module increments the address to the next 16-byte aligned address and starts a PFM access.

The predictive prefetches, like all PFM read accesses, are never aborted. If a new address request does not match the predicted address, a new PFM access occurs after the current access finishes. The PREFEN [1:0] bits (CHECON[5:4]) can start a predictive prefetch. This allows the cache controller to speculatively fetch the next 16-byte aligned set of instructions. The predictive prefetch feature is available only for CPU instruction but not for CPU data and peripheral.

If the selected system clock speed is sufficiently low enough to access the Flash at zero Wait states, the predictive prefetch is detrimental and may be disabled.