[ST25DV64KC] FTM doesn't generate a GPO pulse when a msg is sent on the mailbox

[pa0grg]

Hello everyone,

I'm working on a STM32U585 application which works on STOP3 mode and on a customized electric circuit. One of the wake up sources is the ST25's GPO which should generate a pulse when a message is sent via FTM when mailbox is enabled. In fact, GPO is configured to generate a pulse on GPO/RF_PUT_MSG event (mailbox is enabled on DYN Reg).

When the microcontroller enters in STOP3 mode, I hold ST25's VCC high so that the tag is not powered (the global current absorption is around 12 microampere). The St25 is powered only when a RF source comes near at it (smartphone, ST25 discovery kit such as ST253911B-DISCO...) and using FTM writes something on mailbox, thus generating a pulse on the GPO and finally waking up the microcontroller. 

Unfortunately, that is not working. I made sure the coupling between RF source and tag was fine, but no pulse is seen on GPO.

If ST25's VCC is held low, the global current absorption goes around 150 microampere and the pulse is generated on GPO after FTM procedure

Any suggestions?

[JL. Lebon · Accepted solution]

Hello,

There are several things that can prevent the GPO to correctly work:

Possible wrong configuration:
- the GPO/GPO_EN and GPO/RF_PUT_MSG_EN bits must both be set to 1 in the GPO configuration byte. 
- be careful: the ST25DVxxK and ST25DVxxKC have different GPO configuration. This is sometime source of wrong configuration as people uses the wrong datasheet.
- you can check that those too bits are set, and also that the corresponding two dynamic bits GPO_CTRL_Dyn/GPO_EN and GPO_CTRL_Dyn/RF_PUT_MSG_EN are set as well.

Possible wrong wiring:
- there are two versions of the ST25DV GPO: CMOS version (push-pull) and open drain version. They can be differentiated in several ways: CMOS version is 12 pins package; open drain is 8 pin package. You can also know from the PRODUCT_CODE with is the byte 2 of the UID (see datasheet for the exact value: JF versions are CMOS, IE versions are open drain.).
- the CMOS GPO version requires an external power supply on the VDCG pin in order to drive the output signal to VDCG (GPO is active high). If you have a CMOS version, please check the VDCG voltage.
- the open drain version requires an external pull-up resistor to an active voltage in order to being able to drive the line to 0 (GPO is active low). If you have an open drain version, please check the pull-up resistor and its associated pull-up voltage,

Best regards.