AN2394

Implementation example with STR711F

7/13

3

Implementation example with STR711F

This section describes a hardware and firmware solution of .WAV file playback implemented

for the STR711F microcontroller and that can be easily tailored to any other STR7/STR9

microcontroller.

The following STR711F peripherals are used:

●

TIM0 in Output Compare timing mode to act as SysTimer. The TIM0 global interrupt

can be linked to IRQ (Interrupt Request) channel0 or to FIQ (Fast Interrupt Request)

channel0. For fast, low latency interrupt handling, FIQ channel0 is used to handle TIM0

interrupts.

●

TIM3 in PWM mode.

●

BSPI0 to communicate with the SPI Flash.

3.1

Hardware description

The electrical schematic of the circuit, which the proposed application refers to is shown in

Figure 3.

An M25P64 SPI-based Flash memory is used to store and send back the .WAV file data to

the ICU. The M25P64 is a 64-Mbit (8M x 8) Serial Flash memory from STMicroelectronics,

accessed by a high speed SPI-compatible bus.

The connection scheme between the microcontroller and the SPI Flash is as the following:

pin S0.SCLK(P0.2) is connected to the Serial Clock pin (C) for data synchronization, pin

S0.MOSI(P0.1) and pin S0.MISO(P0.0) are connected, respectively, to Serial Data Input pin

(D) and Serial Data Output pin (Q) for data transmission. To select and deselect the Flash,

its Chip Select pin (S) is dynamically driven by P1.1 pin.

The T3.OCMPA pin (P1.2) is used to output the PWM signal. This signal is passed through

a simple low-pass RC (R33 and C15) filter. The cut-off frequency of this filter is set at 4 kHz.

This ensures that almost the entire audio spectrum is passed through, but the PWM carrier

is cut off.

Since the cut-off frequency of the low-pass filter is set at 4kHz, the R and C component

values are calculated as: R = 1.2 k

Ω, C = 33 nF.

The filter output is fed to the input of a common TS4871 audio amplifier through a

potentiometer to allow continuous control of the audio volume.

The TS4871 is a monolithic integrated audio power amplifier chip from STMicroelectronics

capable of delivering 0.5W of output power into an 8ohm load at 3V3. The amplifier output is

used to drive an 8ohm speaker.