MT8840

5-7

Figure 5 - Smart Telephone Set Block Diagram

Figure 6 - Crystal Oscillator Connections for Driving Multiple MT8840s

Tx

Rx

Summing

Circuit

Telephone

line

MT8840

Micro-

processor

UART

Tx Data

Rx Data

Low-

Pass

Filter

Handset

(Optional)

2W/4W

Converter

When a single crystal is shared among a number of MT8840 devices, OSC1 and OSC2 should be a.c. coupled with a 1000

pF capacitor as shown above. This capacitor is not needed between the device with the crystal and the first driven device.

A capacitor should be used in the first stage whenever such a chain of devices is driven from a clock instead of a crystal. A

33pF capacitor should be connected between OSC1 and VSS to compensate for the load on OSC2.

OSC1

OSC2

MT8840

C=33pF

MT8840

OSC1

OSC2

C=1000pF

MT8840

OSC1

OSC2

C=1000pF

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