Electronic – NRF24L01 presence detection

I have now managed to fix this problem; for posterity I'll explain the solution here.

There turned out to be two problems, neither of which was in fact visible from the config dump pasted above. The configuration is correct. The problems were:

1. The auto-retransmit delay of 1500usec is fine for tiny packets, but my example program was sending 16-byte packets, for which this delay is too small. Sending a single-byte packet works fine.

2. The nRF24L01+ needs a power-decoupling capacitor attached close to the chip, as the long power lines in use in my case (laptop -> USB cable -> Bus Pirate -> bus pirate probe cable -> nRF) meant too much of a power dip at just the moment it turns on the transmitter.

Having attached this cap and using 1-byte packets, it seems all to be working fine.

Reading-While-Write has some limitation in case of Multiple programming operation. See RM0377 § 3.3.4 Writing/erasing the NVM - Program half-page in Flash program memory page 82:

When a half-page operation starts, the memory interface waits for 16 addresses/data, aborting (with a hard fault) all read accesses that are not a fetch (refer to Fetch and prefetch). A fetch stops the half-page operation. The memory content remains unchanged, the FWWERR error is set in the FLASH_SR register.

This means that no fetch operation are allowed in the flash while feeding the memory interface with the 16 words (whatever the code is running in bank 1 and the half page programming is performed in bank 2) . Once the 16 words have been sent to the memory interface, you can resume execution from flash, i.e. during the physical writing of the data inside the flash bank (assuming your code is running in the other bank).

In a nutshell, you need to execute from SRAM the feeding of the memory interface with the 16 words and make sure the interrupts will not cause fetch in flash memory (either mask all interrupts or relocate interrupt handlers and interrupts vector in SRAM).

Note: there is no such limitation in case of Single programming operation.