AN2841

Achieved values

25/31

4

Achieved values

Dynamic computing takes a lot of time and 120 Hz can be reached only for 32 LEDs. In

addition, no other foreground or background tasks can be performed during computation.

As a consequence, this solution is recommended only for simple applications that use only

a few diodes. For other more complicated applications, it is recommended to use the lookup

table approach using a single interrupt only or interrupt + DMA (if DMA is available).

The LED brightness resolution is limited by the size of the lookup table (Table 10). For every

level of brightness, you need a single string of zeros and ones for all the LEDs.

From Figure 13, the maximum refresh frequency can be computed. The time between two

interrupts is 6 µs. This is a refresh rate for one brightness level, for 32 LEDs, 166 kHz.

Table 11. shows the possible refresh rates for whole brightness depth and different LED

count.

Table 10.

Lookup table size (full and reduced)

Bit depth

LEDs

1.

Values indicates the maximum size.

832

256 × 4 = 1 kB

32 × 4 = 128 B

12

32

4096 × 4 = 16 kB

32 × 4 = 128 B

8

128

256 × 16 = 4 kB

128 × 16 = 2 kB

12

128

4096 × 16 = 64 kB

128 × 16 = 2 kB

8

512

256 × 64 = 16 kB

256 × 64 = 16 kB

12

512

4096 × 64 = 256 kB

512 × 64 = 32 kB