Introduction

A handset is the part of a landline telephone that contains a speaker to receive audio, a microphone to transmit audio, and circuitry to interface with the base station. It can also be in the form of a headset.

Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz.

This project will present the design of an SPI digital ISDN telephone handset using two SLG46140V GreenPAK IC's. The designed telephone handset contains a speaker and a microphone and can transmit and receive audio over the SPI bus to/from the base unit at 64 kbit/s (8-bit audio sampled at 8 kHz sampling rate).

The following sections will show:

● the SPI digital telephone handset circuit;

● the SLG46140V GreenPAK telephone handset design in detail;

● how to drive the handset over SPI.

Below we described steps needed to understand how the SPI digital ISDN telephone headset has been programmed. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the device.

SPI Digital Telephone Handset Circuit

Figure 2 shows the handset circuit. The capacitive microphone is connected between GND and a 10 k pull-up resistor to 5 V. This fills the microphone capacitor with a charge proportional to its capacitance. Sound vibrations make the capacitance fluctuate which in turn results in voltage fluctuations on the positive microphone terminal pin. The sound modulated voltage gets AC coupled and transmitted across the 1 uF capacitor to the PGA (+) - input on PIN6 of the left SLG46140V GreenPAK device. The two 10 k resistors connected to PIN6 are a weak voltage divider dividing the
1200 mV VREF on PIN3 to 600 mV to center the input to the middle of the PGA/ADC range (0~1200 mV). PIN7 is the PGA (-) - input and is just centered at 600 mV by two 10 k resistors dividing VREF. That way we have a ZERO point from which we can use the differential PGA amplification (16x) to amplify the microphone signal around the 600 mV ZERO point. Without the differential PGA mode, 16x amplification would result in amplifying also the 600 mV offset, saturating the PGA to the maximum value, and losing the microphone signal.

The microphone signal gets amplified, digitized, and transmitted over the SPI bus to the base unit.

From the base unit, we receive the digital voice signal of the other party over the SPI bus, and inside the right SLG46140V GreenPAK device, it gets converted to a PWM signal (0/5 V, 106 kHz) suitable for the n-MOS driver of the speaker. The n-MOS driver modulates the +5 V voltage on the speaker with the PWM voice signal, creating audible sound on the speaker to hear the other party. The sound is RC filtered for improved clarity.

Figure 3 shows a simple method to generate the PWM pulse train corresponding to a given signal is by using the intersective PWM method: the signal (here the red sine wave) is compared with a sawtooth waveform (blue). When the latter is less than the former, the PWM signal (bottom) is in a high state (1). Otherwise, it is in the low state (0). We do all this digitally, with a counter for the sawtooth, and an updating SPI voice value for the signal.

GreenPAK Design Schematics

The schematic of the GreenPAK design #1 – SPI ADC is shown in Figure 4.

The schematic of the GreenPAK design #2 – SPI DAC is shown in Figure 5.

GreenPAK Design #1 – SPI ADC

The analog microphone signal gets differentially amplified in the PGA (x16) and then gets fed into the ADC to be digitized. The digitized signal gets transmitted over the MISO line (PIN12) while the SPI master...

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