Electronic – STM32F4 interfacing with NRF24l01+

The easiest way would be with the nRF24L01. These modules cost on ebay 1€. with these you can exchange data in higher speed and lower cost. You connect them via SPI, like with the RFID Module.

This is one of the most known designs out there. There are also many other designs with a PCB antenna or with an external one. Range with the Black one is about 60 Meters open Air with 8 bytes/s.

After I had invested many, many hours into problem analysis, I wrote the above question – only to figure it out about one hour later.

The problem turned out to be the SPI timing, more specifically the time between the falling edge of the SPI clock (SCK) for the last bit of the transaction and the rising edge of the chip select (CS). It was 0 but should be at least 2ns.

Invalid SPI timing for NRF24L01+:

Fixed SPI timing:

In the code, it's just one additional line, namely the line setting cs_ena_posttrans:

spi_device_interface_config_t device_config = {
    .clock_speed_hz = 1000000,
    .mode = 0,
    .spics_io_num = CSN_PIN,
    .command_bits = 8,
    .cs_ena_posttrans = 2
};

This not only fixes the FLUSH_TX command; transmitting packets works as well.