Rate Control

Zynq UltraScale+ Device Technical Reference Manual (UG1085)

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2.3 English

Each DMA channel can be independently programmed to issue transactions on a periodic basis. Higher priority channels can have a shorter interval between transactions. The lower priority channels can have a longer interval between transactions. The issue rate is independently controlled for each channel using an interval count that is programmed into the ZDMA_CH_RATE_CTRL [CNT] bit field. Rate control is enabled by setting ZDMA_CH_CTRL0 [RATE_CTRL] = 1. There are 16 pairs of registers for rate control (8 channels in each LPD and FPD DMA unit).

Enabling rate control causes the DMA channel to copy the interval count, ZDMA_CH_RATE_CTRL [CNT] bit field, into the channel's decrementing counter. This counter is decremented with every clock cycle. When the counter reaches 0, the DMA channel issues a transaction to the arbiter and again copies the interval count into the decrementing counter. The channel waits for the counter to reach 0 again, and then issues another transaction and reloads the counter. The cycle continues until disabled by setting [RATE_CTRL] = 0.

Arbitration has no queue of its own and does not get full. Instead, it relies on the AXI queues. Bandwidth maximization depends on parameters such as transaction, length, and alignment.

Rate control remains in effect until disabled. Therefore, when any new transactions begin, the previously programmed rate controls take effect (unless disabled).


TIP:   When rate control is enabled, the read data transaction frequency is always equal to or less than the programmed rate control frequency (1/rate control count).