Implementation example using the ST10 DSP library

AN2182

12/18

7

Implementation example using the ST10 DSP library

7.1

Overview

In this example, an input signal is fed to an analog input of the ST10F27x, processed by a

digital low-pass filter and then output on a PWM output setup as a digital to analog converter.

The digital filter is a low-pass filter with a cutoff frequency equal to 2 kHz and an order of 14

implemented on the ST10F27x using the DSP library provided by STMicroelectronics.

Figure 8.

Low-pass filter modules

The whole implementation includes the following elements:

●

ADC (analog-to-digital converter): the 10-bit ADC of the ST10F27x is used. The input

signal is entered on channel 0 (P5.0) of the ST10F27x. The ADC conversion time is

4.85 µs (ADCST = ADCTC and ST10 frequency = 40 MHz). When the conversion is

complete, the result stored in the ADDAT register is provided to the FIR filter module.

●

Digital filter module: it provides a digital filtered sample. The function used in this

application is the fir_q15_q15_q15() provided in the LibST10.h. This means that the

inputs, outputs and the FIR coefficients are fractional between –1 and 1 and are coded in

the format Q1.15.

●

DAC (digital-to-analog converter): this module converts the digital filtered signal into an

analog one. An 8-bit DAC is implemented using the ST10F27x PWM and an analog low-

pass filter RC. The PWM frequency is 158.8 kHz. The RC filter is used to retain the

continuous components and to remove the frequency of the PWM module. In our case, the

RC cutoff frequency is 26.8 kHz. The digital filtered sample sets the pulse width of the

PWM signal.

ST10F276

ADC

module

Low-

pass

Signal

sampled

PWM

module

P5.0

P5.0

P7.0

27 k

Ω

filter

Signal

filtered

DSP