[Openec] SPI Flash Fast Read, 46% speed increase

[Frieder Ferlemann]

Hi,

it probably is possible to use an SPI flash with double the clock
it currently is used with (64 instead of 32 MHz).
(the Spansion S25FL008A flash on my OLPC is specified for 50 MHz, but
others like ST M25P80 are fit for higher frequencies 75 MHz)

As dwmw2 on IRC mentioned changing might be too late for production.

Switching to the out of spec frequency showed a speed improvement
of about 46% (openec code main loop).
The current mode currently allows for a bus bandwidth of about
4 MByte/s to the EC code memory (plus about 1000 ns setup time
(32 MHz, 32 clocks) for a new address) while the Fast read mode
would double this to 8 MByte/s (plus a setup time of 625 ns
(64 MHz, 40 clocks) for a new address).

The quicker EC translates into powersavings if the XO waits
for the EC and if the SPI clock is properly gated (CLKCFG bit 0,
openec will take advantage of this).

I cannot currently oversee what that speed increase would mean
for the XO.
Yet if a quick SPI is among the options (it would not help
if f.e. a quick SPI had ElectroMagnetic Interference problems)
and it's not too late then one could go for it?)

Greetings,

Frieder