W681308
XXXX PRODUCT DESCRIPTION
The signal from the digital receive input is filtered through the acoustic echo cancellation filter and subtracted from the high
pass filter output. The acoustic echo cancellation is only active in speaker phone operation with 8 kHz sampling. The result is
then passed to the AGC with programmable time, release time and enable signal. The output of the AGC can be passed
either to the PCM interface, the USB/MCU or both (recording a conversation).
Receive Path Operation
The PCM input can be obtained through a multiplexer from the PCM interface or the USB FIFO/MCU. The digital signal can
then be gained or attenuated through a programmable digital gain stage. Then, the side tone from the transmit side is added
through a programmable side tone gain stage. The side tone is disabled when the speakerphone is active.
The digital signal is then passed through a high pass filter with programmable enable. The output of the high pass filter goes
through the interpolation smoothing filter, which produces a 4 bit binary to 15 bit thermometer digital representation for the
sigma delta DAC. The output of the DAC goes through an analog smoothing filter. The output of the smoothing filter can be
hooked up to the speaker phone speaker driver, the earphone speaker driver or both.
A programmable attenuation switch is used to switch between the earphone driver and the speaker phone driver. At power
up of the analog section, the slow ramp on pin VREF1 is used to control the ramp up of the speaker and earphone driver in
order to avoid ‘POP’ sounds. During operation, the user should lower the volume of the speaker using the software volume
control settings, before switching the speaker and earphone driver in order to reduce the ‘POP’ sounds.;
Alternatively, a buzzer can be used on the speakerphone driver outputs, using a 200Hz- 32 kHz PWM signal. However, the
speakerphone can not be used in that case.
Digital CODEC
The digital CODEC filter chain is so designed that it can handle 48K, 16K and 8K rate through its rate change filters.
13.3
Microphone Interface and Auxiliary Interface
W681308 integrates a fully programmable microphone interface. No external components other than the microphone are
required to operate the circuit. The microphone interface can operate in three modes:
Voltage gain mode
Current gain mode
Auxiliary input mode
For the Current gain mode an internal or external resistor can be selected to determine the gain. The Auxiliary input mode
should be used with external resistors. However, an internal gain resistor can be selected. The interface modes above can
be selected with register MIC_MODE [2:0] at address 0x1489.
Voltage Gain Mode
The basic operation is shown below in
Figure 9. The microphone is connected to the pins MCP and RGND. It is important that the negative terminal of the
microphone is routed separately to the RGND pin for ‘pseudo differential’ operation, reducing the background noise amplified
by the microphone amplifier. This can be enforced in the PCB layout by placing a 0 Ohm resistor or a ferrite bead. The pin
MCO is connected to the output of the microphone amplifier and can be used for monitoring the AC level. The voltage gain is
set by register MIC_GAIN[7:4] at address 0x1489. This register provides a gain range from 14dB to 38dB. The gain is set by
a ratio of internal resistors, providing accurate gain control. The pin MCP also supplies the bias reference for the microphone.
The bias consists of a programmable resistor and a programmable voltage reference. The programmable resistor is set by
register MIC_RES[7:4] at address 0x1488 and can be set to open and 670 Ohm to 10kOhm. The programmable voltage
reference is set by register MIC_BIAS[2:0] at address 0x1488 and can be set from 1.22Volt to 2.74Volt.
44
Rev1.2
NUVOTEM TALEMA [ NUVOTEM TALEMA ]
