MCUSEL write ignored

[dmitry.gr]

Code is on APP processor. It is in secure mode, verified using this method: https://community.arm.com/support-forums/f/architectures-and-processors-forum/13548/for-armv8-m-is-there-a-system-register-indicating-the-security-state-of-the-core

So i try to set a pin to peripheral mode, it is ignored...

	NRF_P1_S->PIN_CNF[8] = t = (NRF_P1_S->PIN_CNF[8] &~ (GPIO_PIN_CNF_MCUSEL_Msk | GPIO_PIN_CNF_PULL_Msk | GPIO_PIN_CNF_DRIVE_Msk | GPIO_PIN_CNF_INPUT_Msk | GPIO_PIN_CNF_DIR_Msk)) |
		(GPIO_PIN_CNF_MCUSEL_Peripheral << GPIO_PIN_CNF_MCUSEL_Pos) | (GPIO_PIN_CNF_DRIVE_S0S1 << GPIO_PIN_CNF_DRIVE_Pos) | (GPIO_PIN_CNF_PULL_Disabled << GPIO_PIN_CNF_PULL_Pos) |
		(GPIO_PIN_CNF_INPUT_Disconnect << GPIO_PIN_CNF_INPUT_Pos) | (GPIO_PIN_CNF_DIR_Output << GPIO_PIN_CNF_DIR_Pos);
	
	printf("CNF=0x%08x, wrote 0x%08x\n", NRF_P1_S->PIN_CNF[8], t);

result is 

CNF=0x00000003, wrote 0x30000003

Docs say that to write this, i need ot be in secure mode, which i am. my code is on the app processor, to which the pin is indeed assigned. what is going on?

[Sigurd]

The PIN_CNF[n] register MCUSEL Description field should have had some information about that there are exceptions. In the "Assigning pins between cores, peripherals, or subsystems" section, there is this sentence here that explains the beahvior.

"If a pin is allocated to a subsystem that cannot access it, the pin stays under control of the application core's GPIO peripheral."

https://docs.nordicsemi.com/bundle/ps_nrf5340/page/gpio.html#ariaid-title2

[dmitry.gr]

Ok, so if i understand that correctly, if i assign a pin to a peripheral using PSEL in a peripheral, say P1.08 to PWM0, then i should see "3" in port1's CNF[8].MCUSEL?

or is it the case that while you defined the value, it can never be seen as such there?

[Sigurd]

dmitry.gr said:

if i assign a pin to a peripheral using PSEL in a peripheral, say P1.08 to PWM0, then i should see "3" in port1's CNF[8].MCUSEL?

Only for the peripherals with dedicated pins. PWM0 does not have dedicated pins, it can be set to use many different GPIOs. Peripherals with dedicated pins are QSPI, SPIM4 and TRACE . And also if you use Low-frequency (32.768 kHz) crystal oscillator (LFXO), this need to be connected to P0.00 and P0.01. In these cases, you will need to set CNF[#dedicated_pin].MCUSEL to Peripheral(0x3). After setting this, you will be able to see the value "3" in CNF[#dedicated_pin].MCUSEL , for these mentioned peripherals.