I am unsure how to properly connect an SPI chip (3.3V I/O) to SPI_MISO: does it need level conversion to 2.5V?
I see 3.3V activity on SPI_MISO when internal flash is in use, however, I expected it be 2.5V since AR9331 GPIO is 2.5V...

Can anyone shed some light on this?

Unfortunately, the datasheet of AR9331 is incomplete, section "7.4 GPIO Characteristics" mentions only the Input High Voltage minimum value, but there is no maximum value. In most cases, maximum value is somewhere in 1 to 1.3 x VDD25. VDD25 originates from LDO25_OUT that gives ~2.62V output - as You can find it on page 308...

I already used it several times with 3.3V devices, but if You would like to be really careful, a simple voltage divider can be used on the return line from device as You can find it on the reverse-enginered schematic of TL-WR703N:


In most cases, the ~2.6V output voltage of AR9331 output lines is enough for 3.3V devices. You can check the min/max input / output values of them in their datasheets...

@gedass: can You please confirm if there is a voltage divider between the MISO pin of AR9331 and SO pin of the flash on the design of Carambola 2 ?

Thanks, I already saw the TL-WR703N schematic and the voltage divider drew my attention. And yes, it works with 3.3V but according to specs it does not look to be a good practice.

In case there is a voltage divider, it would be also nice to know if SPI_MISO is connected directly to AR9331 or the flash SO pin (which would explain why I can see 3.3V levels with scope on SPI_MISO).

Waiting for confirmation from gedass.

BTW serial port also must be connected via voltage divider.LAN leds too.But I have never seen module damaged for this reason.

OK, if that is only a 22R, I assume there is no problem adding a voltage divider (3K, 1K) for the external SPI chip to reduce possible risks.