AN2841

Dimming application using an STM32 microcontroller

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More examples of TotalTime for different number of LEDs and PWM resolution can be seen

in Table 6.

Table 6 shows the time required for processing one PWM step, one buffer load and one

buffer latch. With an 8-bit PWM resolution, 256 buffer latches are needed; the maximum LE

(Latch Enable) frequency must be divided by 256. These results are shown in Table 7.

.

Table 7 shows the maximum refresh frequencies for controlled LEDs. But the inverse

computation must be used to solve our task. Table 8 shows the inversion of the Table 7

values and gives the required refresh frequency. The LE frequency must be derived and the

SysTick timer preload value (SysTick-preload-value) set to generate the LE pulse by

generating an interrupt at every counter reload event.

Table 7 and Table 8 show that SysTick-reload-frequency will be set between 25.6 kHz and

371.7 kHz according to requested PWM resolution and LED count.

The STM32 SysTick timer uses a reference clock of 72 MHz when set to the maximum

value. The interrupt period for this timer is the SysTick-preload-value multiplied by 1 / (72

000 000).

The reference clock of STM32 evaluation board is set to 48 MHz for the USB to operate

correctly. This value should be taken into the account while making measurements on this

evaluation board as their computation will influence Equation 3 .

The reference clock can be also divided by 8 by using the hardware divider. The divided

reference clock is then 9 MHz. In this case, the interrupt period for this timer is equal to

a preload value multiplied by 1 / (9 000 000).

Preload value = requested SysTick interrupt time / reference period

Preload value = reference frequency / requested SysTick interrupt frequency.

Table 6.

TotalTime necessary for requested dimming parameters

LEDs

PWM

TotalTime

Max. LE frequency

8

8-bit

2.25 + 0.055 × 8 = 2.69 µs

371.7 kHz

8

12-bit

2.25 + 0.055 × 8 = 2.69 µs

371.7 kHz

672

8-bit

2.25 + 0.055 × 672 = 39.21 µs

25.5 kHz

Table 7.

Maximum refresh rate

LEDs

PWM

Max. LE frequency

Max. refresh rate

8

8-bit

371.7 kHz

1452 Hz

8

12-bit

371.7 kHz

90.7 Hz

672

8-bit

25.5 kHz

99.6 Hz

Table 8.

SysTick-reload-time for 100 Hz

LEDs

PWM

Requested refresh rate

SysTick-reload-time

8

8-bit

100 Hz

39 µs (25.6 kHz)

8

12-bit

90.7 Hz

2.7 µs (371.7 kHz)

672

8-bit

100 Hz

39 µs (25.6 kHz)

32

8-bit

100 Hz

39 µs (25.6 kHz)