Choosing GPIO Speed Setting

[Uwe Bonnes]

Posted on June 19, 2015 at 13:27

Hello,

what is a sensible relation between the GPIO speed setting and peripheral device speed? E.g. OSPEED = 00 on a >2.7 Volt STM32F42x with 10 pF load has a value of 8 MHz max. Is that good for a SPI clock rate of 8 MHz? Or should OSPEED be  substantial higher than the peripheral clock speed? What strategy  to use?

[waclawek.jan]

Posted on June 19, 2015 at 14:02

Frequency is such a dubious measure when it comes to output waveforms. You should base your judgement on the rise and fall times, rather.

JW

[stm322399]

Posted on June 19, 2015 at 14:16

The section 6.3.17 of the F427 datasheet is fairly clear. The maximum frequency F is defined so that rise plus fall times of a square signal last less than 2/3 of the signal period.

Whether you can use 8MHz when IOSPEED is set for 8MHz max, probably also depends on the device you address. IMHO it is safe to switch to the immediately higher IOSPEED class. Of course, the faster you generate signal, the more critical is your impedance adaptation, grounding and all other EMC design rules. What should be absolutely avoided is to set 50 or 100MHZ IOSPEED for a 400KHz I2C signal. I have seen enough of this, not only on this forum !