AS2540 datasheet(9/19 Pages) AMSCO

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Data Sheet AS2540

austriamicrosystems

Revision 3.1

Page 8 of 18

<Line Event> Message

<LineEvent> message format

MSB

11111

LSB

Status

Remark

valid ring signal detected

no ring signal

manual hook switch - off-hook (pin MHS)

manual hook switch - on-hook (pin MHS)

electronic hook switch - off-hook (pin EHS)

electronic hook switch - on-hook (pin EHS)

Status of AS2540

Protocol

Line Events

AS2540 has 3 inputs, namely MHS (Manual Hook Switch),

EHS (Electronic Hook Switch) and RFD (Ring Frequency

Detector), which sense the state of the two hook-switches

and discriminate an applied ring signal. The MHS is

connected to the manual hook switch which is operated by

the handset and the EHS is connected to an electronic

hook switch which is operated by the CPU. Whenever the

state of these inputs changes, AS2540 immediately sends

<LineEvent> to the CPU after a debounce time. MHS and

EHS up-debouncing lasts 15ms, down-debouncing lasts

85ms. RFD start to discriminate the ring frequency

immediately but will send a <LineEvent> message with a

delay of 102ms. A status change of MHS and EHS pins

must not change the momentary selected path of the

speech network.

The <LineEvent>-message is sent without consideration of

the momentary activities of AS2540 in order to meet PTT

requirements (e.g. line break signalling). The code for the

<LineEvent>-message is chosen with regard to the

minimum current consumption in order to operate the

optocouplers. Pending <Ack>-messages will be queued

after the <LineEvent>-message. An already running

transmissions of <Ack> will not be interrupted and

<LineEvent> will be sent afterwards in this case.

Mode transitions

Idle

→

→ BM →

→

→ MPM →

→

→ Idle

AS2540 always starts up in backup mode and is prepared

to act like an independent dialler. <LineEvent> is sent to

the CPU and AS2540 remains in back-up mode until any

valid message is sent from the CPU. This makes sure that

a non-operable CPU does not disable basic dialling

functions.

Mode transition

MPM

→

→ BM

A watchdog timer is started after <LineEvent> and is reset

whenever AS2540 receives a message from the CPU via

the serial interface. If the CPU does not send any further

valid message within 2400ms, AS2540 goes back into

back-up mode, immediately locks the UART and remains in

backup mode till next on-hook.

Dialling Functions

DTMF

The DTMF generator provides 8 frequencies (in Hz):

697+1209

697+1336

697+1477

770+1209

770+1336

770+1477

852+1209

852+1336

852+1477

***

941+1209

941+1336

941+1477

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