Specifications

7.4. SOFTWARE DESIGN

conﬁguration, including GSM SIM card PIN number, the address of the

SMTP server, GPRS conﬁguration and details of attached peripherals. Refer

to Appendix C for a complete list of stored data, their types, sizes and

locations. These settings are restored upon initialisation and checked for

consistency before the conﬁguration is accepted. See Section 7.5 on reliability

for further details.

7.4.2 Real Time Clock

The PIC 18F series has a second low power oscillator based around Timer1.

This is clocked at 32.768kHz and divided down by counting 32768 cycles

using Timer1 to produce a high priority interrupt every second. The ISR then

increments a “seconds” global variable, overﬂowing into minutes, hours, days,

months and years. Leap years are not presently catered-for. This interrupt

must be at the highest priority as the timer counter value must be reset to

32768 as soon as the counter overﬂows to create a one second delay. If the

timer were not preloaded as soon as the interrupt were generated, the system

would lose time. To account for tolerances in the RTC crystal, a timing

correction factor is implemented. This value may be set by the operator and

is added to, or subtracted from the preload value (32768) at every interrupt.

In this way, the clock can be manually sped-up or slowed-down should it be

necessary.

7.4.3 User interface

The display consists of a two line LCD display with 16 characters per line.

This display ordinarily displays the current time on line one and the system

temperature and battery status on line two. When another event occurs

(such as a peripheral poll or GSM uplink activation), the display changes

to reﬂect the progress of these activities. The GSM FSM has the highest

priority and will display its connection status irrespective of other FSMs’

states. Assuming the GSM connection is idle, the peripheral polling FSM has

the second highest priority and it will display the status of polled peripherals

on line two only (keeping the current data/time display on line one).