Why the dummy cycles needed for every APB transfer

Hello,

The way APB transfer is defined, on cycle:

1. sets SEL=1, sets control signals (address, data, write...etc)

2. sets ENABLE=1, keep control signals static

3. assuming device was ready to accept the data ready=1, sets SEL=0, ENABLE=0

4. sets SEL=1, sets new control signals (address, data, write...etc)

5. Jump to cycle#2 and continue from there

The way the standard sequences the events, it will take 3 cycles for a single transaction.  

What benefit does it give to add an extra cycle between SEL and ENABLE signals? Why could not they get asserted on the same cycle and that would make a single transfer within one cycle only (assuming there is no back-pressure). If the subordinate cannot process the data within a cycle it has a way to extent the cycle by keeping the ready at low.

What benefits does it give to push SEL=0 in cycle #3? Why could not I keep it active for back to back transfers?

Thanks in advance,

Khach

Parents Reply Children
No data