SA589 FAMILY
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)neitherkeywillbeaccepteduntilbothkeysarereleasedandthecorrectkeyispressed
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.
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