SA541 Datasheet, PDF (14/16 Pages) Sames – PULSE/DTMF SWITCHABLE DIALLERS

SA541
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 if the device is to be unlocked. This feature
DD
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 150KΩ 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.
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