The bias for the internal microphone is generated from the PSCLD, N301 analog output, VA using a bias generator. The bias generation is designed in such a way that common mode signals induced into the microphone capsule wires are suppressed by the input amplifier in the audio codec. The bias generator is controlled by the MCU to save power, the control signal is taken from the audio codec, N200 output latch, pin 21, when the microphone is not used, in idle the bias generator is switched off. The microphone amplifier gain is set by the MCU to match with the used microphone, 35 dB. The microphone amplifier input to the audio codec is a symmetrical input.
The microphone signal is connected to the baseband using filtering to prevent EMC radiation and RF PA signal to interfere with the microphone signal. L201
Technical Documentation System Module and C201 forms the first part of this filter. R203 and C202 forms the second part of this filter. A similar filter is used in the negative signal path of the microphone signal. R205 is connected in the ground path for the microphone bias current. R202 supplies the bias current to the microphone from the generator circuitry R201, C200 and V200. A transient suppressor, V204, is connected across the microphone terminals to protect the microphone against ESD.
The earpiece amplifier used for the internal earpiece is of differential type and is designed as a bridge amplifier to give the output swing for the required sound pressure. Since the power supply is only 3V a dynamic type ear piece has to be used to achieve the sound pressure. This means that the ear piece is a low impedance type and represents a significant load to the output amplifier. Series inductors are implemented to prevent EMC radiation from the connection on baseband to the earpiece. The same filter also prevents the PA RF field from causing interference in the audio codec, N200 output stage to the earpiece.
The buzzer is controlled by the PWM output provided by the audio codec, N200. Transistors V201 and V202 acts as amplifier and, impedance conversion for the low impedance buzzer. The buzzer is driven directly from the battery voltage. As the buzzer is connected to the baseband via the keyboard the battery voltage provided by VBKEY and the buzzer driving signal BUZZER are EMC protected. As the buzzer is a dynamic one the impedance shows a clear inductance. Therefore a free running diode V203 is used to clip the voltage spikes induced in the buzzer line when the buzzer is switched off.
The buzzer frequency is determined by the internal setup of N200. The frequency is determined by the MCU via the serial control bus. The output level can be adjusted by the PWM function which is attached to the buzzer output in N200.
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