ISO
2
-CMOS
MT8840
Tx
MT8840
Summing
Circuit
Rx
Tx Data
UART
Rx Data
Micro-
processor
Telephone
line
2W/4W
Converter
Low-
Pass
Filter
(Optional)
Handset
Figure 5 - Smart Telephone Set Block Diagram
V
DD
OSC1
MT8840
OSC2
C=33pF
MT8840
MT8840
OSC1
OSC2
C=1000pF
OSC1
OSC2
•••
C=1000pF
V
SS
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.
Figure 6 - Crystal Oscillator Connections for Driving Multiple MT8840s
5-7
MITEL [ MITEL NETWORKS CORPORATION ]
