Specifications

7.4. SOFTWARE DESIGN

of approximately 50 possible states.

Initialisation

Ordinarily, the GSM state is “powered down”. Any software component may

invoke the GSM unit by changing this state to “powering up”. At this point,

the GSM FSM pulls the hardware power line high and continually sends the

module a software reset command (“ATZ”) while waiting for a response. If

no response is received w ithin four seconds, then the power down sequence

is begun. If, however, the unit is correctly functioning, an “OK” response is

received and device initialisation follows. Extended error reporting is enabled

to aid troubleshooting and the status is displayed on the LCD screen on line

two. The reporting mode is also changed from the default “verbose” to

“numeric” w hich results in less processing overhead.

If the module accepts the SIM PIN number (as restored from the EEPROM

backup), responds to all initialisation commands with an “OK” and connects

successfully to either the home network or another network with which the

operator has an agreement, then the state changes to “GSM idling”. At this

point the application which started the GSM module may request an SMS

check, initiate GPRS attach procedure or shut down.

GPRS Attach

The GM862 features an on-board TCP/IP stack. If the software requests

a GPRS attach, then the module attempts to attach to the GPRS APN as

speciﬁed by the user in the EEPROM setting and obtain an IP address. If

this was successful, the obtained IP address is reported to the user via the

LCD display and the GSM state changes to “GSM idling, GPRS attached”.

Once in this state, the host application may initiate an email send. If the

attach was not successful, the FSM will retry the connection. The requesting

application may set the number of retries by pre-setting the global variable

“retry

counter” with the maximum number of permitted retries. With every

retry, this counter is decremented until it reaches zero at which point the

GSM FSM cancels the request. A time-out is also implemented to ensure

that the module does not wait indeﬁnitely for a successful attach.