SA545
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) and 10 Repertory stores are 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 and Repertory stores may be corrupted, it will
always, under all conditions, clear the stores when power is restored, so that corrupt
data is not retained.
Hookswitch Operation
The HSW input is used to inform the SA545 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 and Repertory stores 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.
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).
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3.
SAMES [ SAMES ]
