SWRU068
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be avoided, a magnet loop antenna could be used. The loop is less sensitive to nearby
objects, but suffers of very low efficiency due to very low radiation resistance. The stub
antenna implemented at the Demonstrator PCB showed better efficiency than a loop of
similar size, even when operated hand-held.
4.2
Controller section
The controller section consists of an Atmel AVR ATtiny12L micro-controller with a few
external components.
4.2.1
RC oscillator
The internal RC oscillator is used as reference time base. The oscillator frequency is
relatively independent of temperature and operating voltage. Its nominal frequency is 1.2
MHz, varying less than +/- 5% over the application temperature and supply voltage
range. A calibration feature utilising a calibration word programmed into the micro-
controller at the factory ensures that the oscillator frequency is equal from unit to unit.
4.2.2
Micro-controller I/O
As the controller is packed in an 8-pin package, it has room for only 5 external I/O lines.
These are combined serving several functions in different operating modes.
The micro-controller has only one interrupt input, PB1/INT0. This input is shared
between the “ON” button and the DIO signal to CC400. In sleep mode the controller
will be awaked by an interrupt when this line is forced low by the push button. In
receive mode the interrupt is used for data decoding. During transmit the pin is
configured as an output.
The table below shows the function of all I/O pins in the various modes.
Signal/pin Program
loading
Demonstrator
Comment
PB0, pin 5
MOSI
LED
PB1/INT0,
pin 6
MISO DIO
“ON” in sleep mode
Interrupt input
PB2, pin 7
SCK
CLOCK
“TX”
PB3, pin 2
STROBE
PB4, pin 3
PDATA
4.2.3
In-system-programming
Using the ATMEL AVR STK500 development board, ATAVRISP in-circuit
programmer or similar devices, the micro-controller can be reprogrammed while in the
circuit. Both the program code memory (flash) and the EEPROM can be reprogrammed.
