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SA2531 A/B/C/E/G/U

sames

The side tone cancellation signal is applied to the STB input.

Line Loss Compensation

When Line Loss Compensation is active the gain of the Transmit and Receive amplifiers

are changed by 6dB in accordance with the DC conditions as measured at Pins LI and

LS. When the LLC Pin is Low this adjustment in gain occurs over the range I

50 mA. When LLC is High the range is 45 to 75mA. Note that these figures apply for R1

= R30

Ω.

When the LLC Pin is open the amplifier gains remain fixed regardless of the Line Current

(see figures 6 & 7).

Dialling Functions

Valid Keys

The keypad of the SA2531 comprises a maximum of 32 keys some of which are provided

to cater for options (such as the two Recall/Flash periods). A Bi-polar scan technique is

used so that the 32 keys are scanned in a 4 x 8 matrix using only 8 pins. Two explanatory

keypad arrangements are illustrated in Figures 2 & 3.

A valid key is detected when one and only one contact closure is detected between a Row

and Column Pin. Key contacts are debounced to avoid incorrect detection.

It is also possible to drive the keypad inputs with a micro controller.

Dial Mode Selection

The default mode (LD or MF) can be selected by the Mode pin. When default LD mode

is selected, a temporary change to MF can be invoked by pressing the * key. The circuit

will revert to LD by pressing the R (or R2) key or by next on-hook.

When MF mode is selected by the mode pin, the circuit can not be changed temporary

to LD but will remain in MF.

Last Number Redial

LNR is a facility that allows resignalling of the last manually dialled number without keying

in all the digits again. The LNR is repeatable.

The current contents of the RAM are overwritten by new entries.

A manually entered number is automatically stored in the LNR RAM. The capacity of the

RAM is 31 digits. If a number greater than 31 digits is entered, the LNR facility will be

inhibited (Until new entries < 32 digits) and further entries will be buffered in a First In First

Out Memory (FIFO).