AN2841 datasheet(19/31 Pages) STMICROELECTRONICS

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AN2841

Dimming application using an STM32 microcontroller

19/31

In the above example, the waveform and program code are described in the following steps:

The generation of the LE after entering the interrupt.

Setting the DMA: source address, data length, and Start flag.

Data is sent automatically by DMA.

DMA interrupt routine is called when the last byte is sent by DMA.

Wait until the data from the SPI TX buffer is delivered to the driver.

(Step 6 can be exchanged with calculations that take the amount of time necessary for

the SPI transfer to be finished).

LE generation.

3.2

Data preparation: redundant and non redundant use of

memory

3.2.1

Full lookup table (redundant)

The static lookup table generation is mentioned in Section 3.1.2. The lookup table

generation procedure is described in more detail in this section.

The lookup table row is computed for every PWM step (8-bit PWM means 256 rows for 256

steps). The C code of the full lookup table generation is given in the example below.

Example using a 4×8 LED matrix with an 8-bit PWM resolution

void generateLookUpTable(unsigned char *LEDmapIN){

unsigned int tmpdepth;

for(tmpdepth = 0; tmpdepth < 0xff; tmpdepth++){

for(DATAchar = 0; DATAchar < 4; DATAchar++){

for(outputBIT = 0; outputBIT < 8; outputBIT++){

LookUp[tmpdepth*4 + DATAchar] <<= 1;

if(LEDmapIN[(DATAchar*8) + outputBIT] > tmpdepth)

LookUp[tmpdepth*4 + DATAchar] |= 0x01;

else

LookUp[tmpdepth*4 + DATAchar] &= 0xFE;

}

Table 1.

LEDmap - requested LED brightness configuration

Output

LED 1

LED 2

LED 3

…

LED i

Duty cycle (brightness)

2 / 2n

6 / 2n

4 / 2n

100%

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