AN3143

STM8S audio output hardware overview

Doc ID 17011 Rev 1

9/13

3

STM8S audio output hardware overview

The hardware solution benefits from the STM8S 16-bit timer with pulse width modulation

(PWM) output which can operate in center aligned mode. Only a few external components

are needed to build an application with audio output, for example, a reconstruction filter is

needed to reduce the high frequency noise in the PWM output frequency spectrum (see

Figure 4).

Figure 4.

Single PWM output

1.

LP = Lowpass filter

The reconstruction filter should be set up

as a lowpass filter with a cutoff frequency close to half

times higher than the sampling frequency. This is to keep an adequate margin in the

frequency band for elimination of high frequency noise commponents by the reconstruction

filter. The higher the band stop attenuation (steepest frequency characteristic transition), the

lower the output noise, and consequently, the higher the resolution.

The achievable resolution (N) of the PWM signal is given by the frequency of the PWM

Equation 2

By evaluating Equation 2, where we have a sampling frequency of 15625 Hz and a 16-MHz

system clock, we can only get an 8-bit resolution. Due to the 16-bit output samples, the

lower byte could be lost. This can be audible with higher quality speakers and a good

reconstruction filter.

To avoid such reduction, a second PWM2 output is used to generate a signal corresponding

subsequently added to the signal corresponding to the higher byte PWM1. Theoreticaly, this

allows the 16-bit resolution to be reached. This principle is described in Figure 5.

STM8S

PWM1

STM8S

PWM1

LP

R

C

2

N

f

HSI

4f

sampling

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