Specifications

7.3. HARDWARE DESIGN
Real Time Clock There should be some means of timekeeping – be this
in the form of a local real time clock or a dedicated IC to which the
microprocessor interfaces.
Processor Selection
After completing the RFID subsystem, the author had experience with Microchip’s
PIC 18F series of 8 bit microprocessors. Also considered were Freescale’s
HCS08 range (which were eliminated in favour of the PIC 18F series due to
the lack of 5V support) and Atmel’s 8 bit microprocessor range (which were
eliminated due to the lack of local support and availability). It was thus
decided to continue on the PIC 18F development platform as the devices
fulfilled all the expected memory and processor requirements. These devices
are low power, high speed (10MIPS at 40MHz) microprocessors with on-
board self- programmable FLASH and EEPROM. Furthermore, the family
offers an extensive set of hardware timers and an optimised C compiler.
Additional features which are used include the 10bit ADC (for battery and
temperature monitoring), programmable USART (for communications with
the GSM module) and an SPI & I
2
C compatible Master Synchronous Serial
Port (MSSP).
The PIC 18F range is available to operate from any voltage between 2.0V
and 5.5V, making them ideal for interfacing to standard logic components
such as LCD displays (many competitors’ similarly performing processors
only operate at 3.3V, thereby requiring level shifters on all 5V interfaces).
The devices are also available in PDIP packages, which eases prototyping.
The PIC 18F4620 was selected as the primary system processor.
The PIC 18F4620 features two oscillators: one is the main system clock
and the second is a low power, low frequency design which is used for the real
time clock. It will be clocked from a 32.768kHz crystal. This crystal should
be a low tolerance device specified over an extended temperature range. An
I
2
C EEPROM device is included for additional non-volatile storage.
7.3.3 User Interface
The user interface should be:
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