Specifications

7.4. SOFTWARE DESIGN

the master (the uplink module ) to a slave (any attached peripheral), followed

by nine data bytes from the slave to the master. It is assumed that the MSB

will be a status indication and the remaining eight bytes are data. For the

purposes of the RFID peripheral, the status byte represents the direction and

speed of the bird and the remaining eight bytes will b e used for the 64 bit

ID. This exchange makes use of I

C’s repeated start facility and Figure 7.4

illustrates a typical e xchange with a peripheral.

Figure 7.4: I

C protocol used in uplink module

Diagram based on illustration from [19].

The byte sent to the slave before retrieving the data is used to send

commands to the device (for example, power down, power up or sleep) and

is also useful if the peripheral device supplies more than one data type (for

example, the same weather monitoring peripheral can supply temperature,

wind speed and direction or atmospheric pressure). This initial master-slave

byte allows for the master to select one of the peripheral’s diﬀerent responses.

7.4 Software De sign

The ﬁrmware runs on a microprocessor with limited processing power and

memory resources. It is thus important that the programming model be

fast, able to handle multiple processes and have a compact memory footprint.

Bearing this in mind, it was decided to use communicating ﬁnite state machines

(FSMs). Communication takes place via shared memory.

The development environment is provided by the manufacturer of the

chosen processor, Microchip’s MPLAB v7.4 with the C18 C-compiler. Included

with this package are interfaces to the processor’s on-board peripherals (such

as timers, communication busses, ADCs) and methods for controlling popular

attached hardware devices (such as I

C EEPROM devices and intelligent

LCD displays).