AN2841 datasheet(22/31 Pages) STMICROELECTRONICS

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Dimming application using an STM32 microcontroller

AN2841

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3.3

Timing: set up of the main dimming timer

3.3.1

The main system timer, SysTick setting

The approach with interrupt + DMA + interrupt is considered the most effective.

The microcontroller can reach the maximum frame rate based on the minimum time

required for the all the necessary operations. A time reserve of 6% was used:

●

tDMAen re-enabling the DMA (~ 1.25 µs)

●

toneSPI time necessary for transfer of the data over SPI (~ LEDs × 55 ns)

●

tLEgen LE generation and new data pointer generation (~ 1 µs)

The total time needed for correct processing is the sum of the times above:

Equation 2

TotalTime = tDMAen + toneSPI × LEDs + tLEgen

= 1.25 µs + 55 ns × LEDs + 1 µs

= 2.25 µs + 55 ns × LEDs

The number of LEDs must be rounded up to nearest number commensurable by the length

of an SPI buffer. The SPI buffer length is 8 either or 16 bits in most cases.

Table 4.

LEDmap - reordered requested LED brightness configuration

Output

LED 1

LED 3

LED 2

…

LED x

LED i

Duty cycle (brightness)

2 / 2n

4 / 2n

6 / 2n

× / 2n

100%

Step of change (time of

change)

100%

Table row

Table 5.

Reduced lookup table

Row

Level

Output

LED 1

LED 2

LED 3

…

LED i

……

m(1)

"m" is less or equal to number of LEDs = i

x + 1

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