ARM Technical Support Knowledge Articles

What is the difference between L2C-310 r3 IDLE and CLKSTOPPED outputs ?

Applies to: AXI PL310 L2CC

Answer

IDLE and CLOCKSTOPPED are equivalent functions. The difference is that in r2, the SOC must
observe the IDLE output to get the clock externally to the PL310. Whereas in r3, the PL310 gates
it’s own clock internally, if the dynamic clock gating option is selected, and asserts CLKSTOPPED
output when it has gated the clock. R3 still has the IDLE output, but it is redundant.

IDLE is asserted high as soon as there is no activity in PL310, whereas CLSTOPPED is asserted
high when PL310 has stopped its clock internally, which takes a certain amount of cycles to
happen once PL310 has detected it is idle. As an example, when PL310 detects it is idle, it needs
to de-assert its READY signals on its AXI slave interfaces before stopping the clock.

It is mandatory to have either dynamic clock gating or standby mode (or both) enabled to see any activity on CLKSTOPPED output.

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