Atmel AVR10004: RCB256RFR2 – Hardware User Manual [APPLICATION NOTE]
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4.2
Microcontroller
The Atmel ATmega256RFR2 integrates a low-power 8-bit microcontroller based on the AVR enhanced RISC
architecture. The 256KB non-volatile flash program memory and 32KB internal SRAM, supported by a rich set of
peripheral units, makes it suitable for a full-function sensor network node.
The microcontroller is capable of operating as a PAN-coordinator, a full-function device (FFD), or a reduced function
device (RFD), as defined by IEEE 802.15.4
. However, the RCB is not limited to these, and can be programmed to
operate in other standards or ISM applications, too.
All spare I/O pins are accessible via the expansion connectors for external use.
The Atmel ATmega256RFR2 is designed to operate at full 16MHz speed over the complete supply voltage range from
1.8V to 3.6V.
4.3
On-chip radio transceiver
Besides an 8-bit AVR microcontroller, the ATmega256RFR2 also integrates an IEEE 802.15.4-compliant radio
transceiver. RF and baseband critical components are integrated to transmit and receive signals according to IEEE
802.15.4 or proprietary ISM data rates.
The RCB illustrates a minimal component count implementation. Filter-balun B1
operates as a differential to single-
ended converter connecting the ATmega256RFR2 to a standard SMA connector. An integrated harmonic filter ensures
sufficient harmonic rejection.
A 2.45GHz ISM antenna must be connected to the SMA connector for proper operation.
Any modification of components, PCB layout, or shielding may influence the performance of the circuitry and cause
existing certifications to be invalid.
4.4
Clock sources
4.4.1
Radio transceiver clock
The integrated radio transceiver is clocked by a highly accurate 16MHz reference crystal, Q2. Operating the node
according to IEEE 802.15.4, the reference frequency deviation must be within ±40ppm (see
). The absolute clock
frequency is mainly determined by the external load capacitance of the crystal, which depends on the crystal type and is
given in its datasheet.
The radio transceiver reference crystal, Q2, must be isolated from fast switching digital signals and surrounded by a
grounded guard trace to minimize disturbance of the oscillation.
The RCB uses a SIWARD SX4025 crystal with two load capacitors of 10pF each. To compensate for fabrication and
environment variations, the frequency can be tuned with the transceiver register, XOSC_CTRL (0x12) (see
). An
initial tuning is done during fabrication, and the correction value has been stored in the onboard ID EEPROM (see
By setting the fuses accordingly, the microcontroller can also be clocked by the 16MHz radio reference crystal.
4.4.2
Microcontroller clock
The Atmel ATmega256RFR2 provides several clock source options for the internal microcontroller:
•
16MHz calibrated internal RC oscillator
•
128kHz internal RC oscillator
•
16MHz radio reference crystal
