AON timer prescale_count is not reset after a new wkup_ctrl.prescaler is written

[antmarzam]

There is a bug in aon_timer_core.

The issue becomes visible in the following scenario:

1. WKUP counter counts (wkup_ctrl.enable=1)
2. WKUP count is disabled (wkup_ctrl.enable=0) leaving prescale_count_q=A
3. WKUP count is enabled again, and a new wkup_ctrl.prescaler=B is written.

If B < A, the RTL will have to overflow prescaler_count_q before the new prescaler count becomes effective.

Assuming the expected use case for the WKUP counter is to set the prescaler and leave it as is until reset this isn't a critical bug.

The proposal is to reset prescaler_count_q every time a new wup_ctrl.prescaler value is written as well as the current reset on rst_aon_ni.

[marnovandermaas]

Alternatively, you can change the equal sign to a greater than or equal sign:

opentitan/hw/ip/aon_timer/rtl/aon_timer_core.sv

Line 59 in 53705b7

(prescale_count_q == reg2hw_i.wkup_ctrl.prescaler.q);

[antmarzam]

Alternatively, you can change the equal sign to a greater than or equal sign:

If we change it so it becomes: (prescale_count_q >= reg2hw_i.wkup_ctrl.prescaler.q); , then that would mean the first counted unit would be incremented without any prescaler increments. Would that be ok?