[Microbit-Python] SPI APIs on BBC micro:bit and ESP8266

[M.-A. Lemburg]

On 21.11.2016 11:01, Radomir Dopieralski wrote:
> On Mon, 21 Nov 2016 10:53:36 +0100
> "M.-A. Lemburg" <mal at egenix.com> wrote:
> 
>> Since both devices were running in SPI master mode, we simply
>> crossed the MISO and MOSI cables, connecting MISO on the
>> ESP to MOSI on the MB and vice-versa. We also pulled down
>> the chip select pin on the ESP to GND using a 4.7k resistor.
>> The MB doesn't expose this pin.
> 
> This won't work. If you do that, each of the boards will completely
> ignore the other board's clock, and just spew data at its own pace.
> It may (unreliably) work when using two identical boards, as they are
> likely to have similar clocks, however, even that will go out of sync
> eventually. With completely different boards, such as the Micro:bit and
> the ESP8266, and with the clock speeds set with limited precision (the
> prescaler for the clock has only so much resolution), you are very
> unlikely to get any synchronisation at all.

Ah, now that explains what we were seeing :-)

Thanks for the explanation.

> You could try using NodeMCU on the ESP8266, it has an I2C slave
> implemented.

Hmm, that would mean programming LUA... I'd rather stay with
MicroPython, if possible :-)

Looking at the mbed sources for SPI, the blocking appears to
be a result of their API implementation:

https://developer.mbed.org/users/mbed_official/code/mbed-src/file/a11c0372f0ba/hal/spi_api.h

(even though it is only documented for the slave APIs)

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