SA589 FAMILY

sames

3/14

Hookswitch Operation

The HSW input is used to inform the SA589 of whether the telephone is on 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 is so that short line breaks of less than 200ms, such as the line reversals applied

by the exchange, are ignored. In this case the IMP and MASK 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 MEMORY 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 going on-hook or signalling a TBR, (see timing diagram). COLUMN

outputs also go high until a key is pressed and are normally off whenever timing functions are

not required.

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. When a key is pressed this connects a

COLUMN output to a ROW input and the ROW input is pulled 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 about 14ms (plus the oscillator start-up time if it was not

already running).

Simultaneous key depressions

If two keys are pressed simultaneously (i.e. a second key is pressed before the first has been

verified) neither key will be accepted until both keys are released and the correct key is pressed

again.

Dialling Mode Selection

The dialling mode may be selected via the SELECT pin (Pin 4) as detailed in Table 2. Four

'Loop-Disconnect' + DTMF options, two 'Loop-disconnect only' options and three DTMF

modes are available. Each mode offers a different combination of LD and/or MF timing. If

one of the 'LD only' or DTMF only modes is selected then dialling will remain fixed in LD mode

or DTMF mode respectively.