19.3.3.2 Arbitration Priority Scheme

The MATRIX arbitration scheme is organized in priority pools.

Round-robin priority is used in the highest and lowest priority pools, whereas fixed level priority is used between priority pools and in the intermediate priority pools.

For each Client, each Host is assigned to one of the Client priority pools through the priority registers for Clients (MxPR fields of MATRIX_PRAS and MATRIX_PRBS). When evaluating Host requests, this programmed priority level always takes precedence.

After reset, all the Hosts except those of the Cortex-M7 belong to the lowest priority pool (MxPR = 0) and are therefore granted bus access in a true round-robin order.

The highest priority pool must be specifically reserved for Hosts requiring very low access latency. If more than one Host belongs to this pool, they will be granted bus access in a biased round-robin manner which allows tight and deterministic maximum access latency from AHB bus requests. In the worst case, any currently occurring high-priority Host request will be granted after the current bus Host access has ended and other high priority pool Host requests, if any, have been granted once each.

The lowest priority pool shares the remaining bus bandwidth between AHB Hosts.

Intermediate priority pools allow fine priority tuning. Typically, a moderately latency-critical Host or a bandwidth-only critical Host will use such a priority level. The higher the priority level (MxPR value), the higher the Host priority.

All combinations of MxPR values are allowed for all Hosts and Clients. For example, some Hosts might be assigned the highest priority pool (round-robin), and remaining Hosts the lowest priority pool (round-robin), with no Host for intermediate fix priority levels.

If more than one Host requests the Client bus, regardless of the respective Hosts priorities, no Host will be granted the Client bus for two consecutive runs. A Host can only get back-to-back grants so long as it is the only requesting Host.