Specifications

7.3. HARDWARE DESIGN

perform audio functions (such as microphone and hands-free connections)

and links to peripherals such as cameras or additional SIM card holders.

Of interest to us is the command interface, which is in the form of 2.8V-

level RS232 signals. Thus, level shif ters will be required on both serial lines to

communicate with standard 5V logic. Although commercial ICs are available

to perform this function, it can also be simply achieved by a pair of discrete

transistors for each line. A recommended circuit is provided in the module’s

datasheet. The circuit was modiﬁed slightly to accept components which

were in surplus at the University’s electronics store.

Power requirements for the GM862 are non-standard: 3.6V to 4.2V

supply with current p eaks of 1.9A. Average current (while connected) is

approximately 350mA.[

17] These peaks are very short-lived (in the order of

microseconds) and have a recurring frequency of approximately 216Hz. The

datasheet recommends a 100µF tantalum decoupling capacitor be used close

to the module. Although linear regulators are recommended, if a switching

supply is to be used, then the switching frequency should be in excess of

500kHz to improve transient response.

Apart from the power connections and RS232 communication lines, the

module also requires a “switch-on” signal which is performed by an open-

collector transistor on pin 17 of the 50pin Molex board-board connector.

Power status indication is provided by another line which is held high by

the GM862 when it is powered-up. Also used is a hardware reset which can

be asserted should the device stop responding to commands. However, this

should be only be used as part of an emergency recovery procedure because it

is contrary to the GSM speciﬁcation (which requires all devices to deregister

from the connected base-station before shutting down or restarting) [17].

The module is a quad-band unit (800MHz, 900MHz, 1800MHz and 1900MHz),

of which two are used in this country (900MHz & 1800MHz). The antenna

should thus be capable of receiving both frequencies. The suppliers of the

GM862 module recommended a low-cost, +2dBi, tri-band PCB antenna

which is designed to be housed within the system enclosure at a cost of

ZAR40.00. The connector would add a cost of ZAR15.00. This cost is

negligible when compared the cost of the GSM module and the rest of the

system. Furthermore, the antenna is small (80mm × 20mm × 1mm). When