Can a low power timer (LPTIM) drive an RGB LED in STOP mode?

[DerekR]

MCU: STM32L4R9ZGJ6

Hardware: Custom battery powered board

LED: ARGB1313HS

Desired LED functionality: Blink, on/off, fade in/out ("Breathing")

Hello,

I am familiar with a regular timer and how they can drive a RGB LED using multiple channels (CCx registers) from the same timer. Unfortunately, it doesn't appear this works in STOP low power mode since the PWM will be clock gated.

Question 1: Can all legs (3 pins) of an RGB LED be driven by a low power timer (LPTIM) in STOP mode? It looks like a low power timer (LPTIM) can only drive 1 GPIO in CubeMX.

Question 2: If the answer above is 'no', then what is the most power efficient way to PWM a 3 legged RGB LED?

Thanks,

Derek

[Peter BENSCH]

Why not just run the STM32L4R9 in low-power run mode at e.g. 100kHz, then it draws only 0.21mA at 25°C, while your RGB LED ARGB1313HS draws about 60mA at 100% duty cycle (RGB=on)?

Regards

/Peter

[DerekR]

@Peter BENSCH​ 

Can you confirm that the LPTIM can only drive one GPIO? I only see one compare register in the datasheet (LPTIM_CMP). Any other way to PWM a RGB LED in STOP mode?

Thanks,

Derek

[Peter BENSCH]

Correct, LPTIM can only output one PWM channel. By the way, the AN4865 describes some use cases for the LPTIM.

Unfortunately, I don't know of any way to output three independent PWMs with the STM32L4R9 in STOP mode. However, this could be done with external hardware, e.g. a small auxiliary Ultra Low Power MCU, for which the STM32L010 could be suitable, taking its clock from the master MCU.

Hope that helps?

Regards

/Peter

[DerekR]

Hey Peter,

The STM is also doing a bunch of other stuff such as driving an AMOLED touch display using TouchGFX, playing audio, reading various sensors over I2C/SPI/UART, etc. The HCLK is running at 120 MHz.

Thanks,

Derek