Technical Manual

10
Hardware
For further details, consult the schematic diagram in Figure 2.
2.1.1. The Modem Interface
The modem operates with two tones: 1 kHz (representing the “1” bits) and 2 kHz
(representing the “0” bits). A bit cell is represented by a complete time period ( 1/f ),
thus the raw throughput varies between 1 and 2 kbps (average 1.5 kbps). The mo-
dem functions are essentially implemented in software. However, a signal condition-
ing is performed on both receive and transmit paths.
On receive, the buffered analog data signal from the radio unit is applied to a 3 kHz
low pass filter (U6). The filter output is further fed both to a Schmidt trigger (U5:A)
and a 100 Hz low-pass filter (U5:B), the output of the latter being used as a reference
for the slicer (the Schmidt trigger). The TTL data, RXDO , is obtained at the output
of the slicer (i.e. on TP1). The microcontroller overtakes the decoding operation (see
also “Modulation Technique Used” on page 57).
On transmit, a two-poles low-pass filter (U11) is used: its role is to “smooth” the
square signal TXDI generated by the microcontroller. Before entering the filter, a
microprocessor-controlled, variable-gain amplifier built with U12/U13 is used to set
the modulation level for different bandwidths (12.5, 20 or 25 kHz). The selected val-
ue is stored in the microcontroller’s EEPROM during the aligning procedure.
2.1.2. The Microcontroller and the Power Management Sections
The operation of the whole unit is under the control of U9, an Atmel ATMega 103
microcontroller. It is a powerful chip exhibiting a very low power consumption. Its
main functions are:
• Controls the radio unit
• Implements the modem functionality
• Assembles the radio frames and waits for requests from a remote
• Performs the sampling of the sensor inputs and the A/D conversion
• Stores the values in a local FIFO; manages the FIFO
• Implements the pulse counter function
• Manages the real-time clock
• Assures the power management
• Implements a serial Command Line Interface (CLI)
The chip operates at its maximum speed, in this case 4 MHz (the “L” version), and
uses a crystal (X2) for the on-board clock generator. The real time clock is imple-
mented by means of a 32.768 kHz crystal (X1) connected on the internal Timer/
Counter0.