Dimming application using an STM32 microcontroller

AN2841

24/31

Equation 3

SysTick-preload-value = 72 000 000 / 25 600 = 2812

The frequency ranges 25.6 kHz and 371.7 kHz required for all possible applications ranges

can be achieved using preload values: 2812 and 194, or with divider by 8: 351 or 24.

SysTick timer setting with no divider on the reference clock:

SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK);

SysTick_SetReload(900);

generates pulses with the period of 12 µs.

SysTick timer setting with the divider by 8 on the reference clock:

SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK_Div8);

SysTick_SetReload(900);

generates pulses with the period of 100 µs.

The setup algorithm for the SysTick timer configuration has to check if it is possible to set

preload value to keep enough time for processing (TotalTime) together with reaching the

requested refresh frequency:

TotalTime = 2.25 + 0.055 × LEDs

SysTick-reload-time = 1 / SysTick-reload-frequency

TotalTime is the time slot, during which the SPI transfer and all the arbitrations needs to be

achieved. The SysTick time is the time slot which must be respected to reach the required

refresh rate.

Equation 4

SysTick-reload-time > TotalTime

To reach the requested refresh rate and set up the SysTick timer preload value:

SysTick-reload-frequency = requested-refresh-rate × PWMdepth (100 Hz × 256 steps).

Table 9.

Example of values used in Equations 3 and 4

LEDs

PWM

Requested refresh rate

32

8-bit

100 Hz

39 µs (25.6 kHz)