ST3917A datasheet(8/16 Pages) STMICROELECTRONICS

8 / 16 page

DEVICE OPERATION

During on-hook all keypad inputs are high imped-

ance internally and it requires very low current for

memory retention. At anytime,row inputs are initial-

ized at logic level 1 and column inputs are initialized

at logic level 0 at off-hook. The circuit verifies that

a valid key has been entered by alternately scan-

ning the Row and Column inputs. If the input is still

valid following 32ms of debounce,the digit is stored

into memory, and dialing begins after a pre-signal

delay of approximately 40ms measured from the

initial key closure. Output tone duration is shown in

Table 1.

The device allows manual dialing of an indefinite

number of digits, but if more than 32 digits are

dialed, LND will be inhibited.

Table 4 : DTMF Output Frequencies (Hz)

Key Input

Std

Frequency

Output

Frequency

%in

Deviation

Row 1

Row 2

Row 3

Row 4

Column 1

Column 2

Column 3

697

770

852

941

1209

1336

1477

699.1

766.2

847.4

948

1215.9

1331.7

1471.9

+0.31

-0.49

-0.54

+0.74

+0.57

-0.32

-0.35

Normal Dialing

D D D ...

Normal dialing is straight forward, all keyboard

entries will be stored in the buffer and signalled in

succession.

Hook Flash

D Flash D ...

Hook flash may be entered into the dialed sequence

at any point by keying in the function key, FLASH.

When a FLASH key is pressed, no further key inputs

will be accepteduntil the hookflashfunctionhasbeen

dialed. The key input following aFLASH will be stored

as the initial digit of the new number (overwriting the

numberdialed beforethe FLASH) unless it is another

FLASH. FLASH key pressed immediately afterhook-

switch or LND will not clear the LND buffer unless

digits are entered following the FLASH key.

Flash

LND not cleared

LND Flash

LND not cleared

LND Flash D1 D2

LND buffer will contain D1, D2

Last Number Dialed

Off-Hook LND

Last number redialing is accomplished by entering

the LND key at off-hook or after the FLASH key, the

subsequent LND keys pressed will be ignored.

Sequence

Digit Emitted

OFF-HO OK, D1, D2, ON-HOOK

D1, D2

OFF-HOO K, LND, LND, ON-HOOK

D1, D2

OFF-HOO K, D3, D4, LND, ON-HOOK

D3, D4

OFF-HO OK, LND, ON-HOOK

D3, D4

Last Number Dialed Inhibition

Last number redialing by LND key is inhibited if

there are more than 32 digits stored.

Last Number Dialed Cascading

Digits dial after the LND will cascade into the LND

buffer for the next redialing. In cascade operation,

the keyboard is inhibited upon pressing the LND

key, the LND output must be completed before

acceptance of any key entry.

Sequence

Digit Emitted

OFF-HO OK, D1, D2, ON-HOOK

D1, D2

OFF-HOO K, LND, D3, D4, ON-HOOK D1, D2, D3, D4

OFF-HO OK, LND, ON-HOOK

D1, D2, D3, D4

Pause

Off-Hook D Pause D ...

A pause may be entered into the dialed sequence

at any point by keying in the special function key,

PAUSE. Pause inserts a 3.1 seconds (Tone mode)

or 3.4 seconds (Pulse mode) delay into the dialing

sequence. The total delay, including predigit and

post digit pauses as shown in Table 5.

Table 5 : Special Function Delays

Function

First / Auto

Delay (second)

Pulse

Tone

SOFT-

SWITCH

First

Auto

0.2

PAUSE

First

Auto

2.6

3.4

3.0

3.1

Each delay shown in Table 5 represents the time

required from the time the special function key is

depressed until a new digit is dialed.

The time is considered ”FIRST” key if all previous

inputs have been completely dialed. The time is

considered ”AUTO” if in redial, or if previous digits

dialing is still in progress.

ST3917A - ST3917B

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