AN2182

Implementation example using the ST10 DSP library

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The following flowchart summarizes the software implementation of the low-pass filter:

Figure 9.

Low-pass filter flowchart

1.

Please refer to the technical note ST10 DSP library for a detailed description of the FIR structure used and its

fields.

2.

If the filter’s order is 14, 15 input samples are needed to compute an output. A circular buffer is used to store

the 15 input samples of the input signal. The size of the circular buffer is 2 x (filter’s order +1), which is 30 in this

example. An input sample is stored twice in the circular buffer: at buffer[i] and buffer[i + filter’s order +1] where

0<= i<= filter’s order.

3.

The ST10F27x ADC converts analog voltages between 0 and 5V into 10-bit digital values. The inputs for the

fir_q15_q15_q15 should be fractional. To normalize these samples, they are shifted to the left.

4.

The FIR function delivers a fractional output coded on 16 bits(Q1.15). The DAC realized with the ST10F27x is

an 8-bit one.

PWM initialization (PP0 = 0xFF)

ADC initialization

PWMIR = 1

Clear PWMIR

Trigger ADC conversion

ADCIR = 1

ADCIR = 0

Apply the filter->Output

No

No

A

B