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11. LOCKING FACILITY

This feature is available on the SA54L1. Pin 11 (LOCK) should be connected to V

SS

if the device is to be locked and to V

can be controlled by an inexpensive low voltage SPDT switch. When locked,

columns 1,2 and 3 on the keypad are disabled making it impossible to dial out any

digits. However column 4 is not disabled, thus allowing the user to redial the contents

of the LNR store. The user would therefore typically store an emergency number

in the LNR memory before locking the keypad.

TYPICAL APPLICATIONS

The circuit in Figure 7 uses a constant current supply to draw current from the

telephone line which is used to power the SA54X1. The 2,5V reference diode

produces a stabilised supply voltage which determines the DTMF tone level of the

SA54X1.

In Figure 8, a stabilising voltage from the speech circuit is used to supply the SA54X1

during DTMF dialling to give accurate tone levels.

The SA54X1 is powered via the 150K

Ω resistor during TBR operations and LD

dialling breaks, and via T1 during dialling makes. This configuration minimises the

component count, at the expense of allowing a leakage current of about 450µA

during dialling breaks. The 47µF reservoir capacitor maintains and smoothes the

supply to the chip.