needs some clarification

PREADY can remain high forever if you don't want to add wait states. If you are connecting an APB2 peripheral (with no PREADY output) to an APB3 master, you would just tie the PREADY input on the master high.

PREADY is only checked during the "ACCESS" phase of an APB access, so during periods when PSEL is low, or when PSEL is high but PENABLE is low (the transfer "SETUP" phase), PREADY is undefined and could be high.

PREADY can remain high forever if you don't want to add wait states. If you are connecting an APB2 peripheral (with no PREADY output) to an APB3 master, you would just tie the PREADY input on the master high.

PREADY is only checked during the "ACCESS" phase of an APB access, so during periods when PSEL is low, or when PSEL is high but PENABLE is low (the transfer "SETUP" phase), PREADY is undefined and could be high.