SA541

sames

7/16

FUNCTIONAL DESCRIPTION

Note: All timing data assumes the use of a 560 kHz ceramic resonator as time base.

1. Power On

When power is applied to the chip, a power-on reset circuit operates and ensures that

the Last Number Redial (LNR) store is cleared and all logic is reset. The power-on

reset circuit is designed such that if the chip supply voltage drops to a level at which

the LNR store may be corrupted, it will always, under all conditions, clear the store

when power is restored, so that corrupt data is not retained.

2. Hookswitch Operation

The HSW input is used to inform the SA54X1 of whether the telephone is on-hook

or off-hook. Logic ‘0’ is recognised as on-hook, Logic ‘1’ is recognised as off-hook.

When the HSW input rises from ‘0’ to ‘1’ the off-hook state is recognised immediately

and keypad inputs are accepted. However, when the HSW input falls from ‘1’ to ‘0’

the on-hook state is not recognised for 200-300ms. This ensures that short line

breaks of less than 200ms, such as line reversals applied by the exchange, are

ignored. In this case, the MASK and IMP outputs will go low immediately the HSW

input goes low in order to preserve current, but will resume normal operation

immediately HSW goes high.

On-hook state: In the on-hook state all chip outputs are set low, the oscillator circuit

is inhibited and no key inputs are accepted. This conserves supply current so that

the LNR store contents may be retained.

Off-hook state: When the HSW input goes high, the MASK output immediately

goes to the logic ‘1’ level and remains there until dialling starts (see Figure 5).

COLUMN outputs also go high until a key is pressed. The oscillator circuit remains

inactive until a key is pressed, and is normally off whenever timing functions are not

required, hence reducing operating current.

3.

Keypad Operation

A single contact, normally open keypad is required. When off-hook, the COLUMN

outputs are normally held high and the ROW inputs are low (via internal pull-down

resistors). When a key is pressed, a COLUMN output is connected to a ROW input,

pulling the latter high.

This action initiates keyboard scanning. During keyboard scanning, the COLUMN

outputs are normally low but generate scanning pulses at 7ms intervals on each

output, in sequence. A key is accepted as valid when two successive scanning

pulses from the same COLUMN are seen on a ROW input. Hence, the minimum

bounce-free key closure period which is necessary to guarantee detection is

14ms(plus the oscillator start up time if it was not already running).