AT02876: Atmel REB212BSMA Hardware User Manual [APPLICATION NOTE]
Atmel-42097B-WIRELESS-AT02876-REB212BSMA-Hardware-User-Manual_ApplicationNote_062014
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All components connected to nets DEVDD/EVDD contribute to the total current consumption.
While in radio transceiver SLEEP state, most of the supply current is drawn by the 1MΩ pull-up resistor R21 and
the EEPROM standby current.
Figure 4-3.
Power Supply Routing
4.4 Radio Transceiver Reference Clock
The integrated radio transceiver is clocked by a 16MHz reference crystal, Q1. Operating the node according to
, the reference frequency must not exceed a deviation of ±40ppm. The absolute frequency is
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, Q1, shall be isolated from fast switching digital signals and surrounded
by a grounded guard trace to minimize disturbances of the oscillation. Detailed layout considerations can be
found in Section
The REB212BSMA uses a Siward SX4025 crystal with two load capacitors of 10pF each. To compensate for
fabrication and environment variations, the frequency can be further tuned using the radio transceiver register
XOSC_CTRL (0x12), refer to
. The REB production test guarantees a tolerance of 20ppm and -5ppm.
The correction value, to be applied to TRX register XOSC_CTRL (0x12), is stored in the ID EEPROM, see
Section
The reference frequency is also available at pin CLKM of the radio transceiver and, depending on the related
register setting; it is divided by an internal prescaler. The CLKM signal is filtered by a low-pass filter to reduce
harmonic emissions within the 868 / 915MHz ISM band. The filter is designed to provide a stable 1MHz clock
signal with correct logic level to a microcontroller pin with sufficiently suppressed harmonics. Usage of CLKM
frequencies above 1MHz requires a re-design of R25 and C40. In case of RC cut-off frequency adjustments,
depending on the specific load and signal routing conditions, one may observe performance degradation within
affected IEEE channels.
Channel 4 (912MHz) is affected by the following harmonics: 57 × 16MHz or 114 × 8MHz.
By default, CLKM is routed to a microcontroller timer input; check the individual configuration resistors in the
schematic drawing (see Appendix
). To connect CLKM to the microcontroller main clock input, assemble R3
with a 0Ω resistor.
4.5 RF Section
The Atmel AT86RF212B radio transceiver incorporates all RF and base band critical components, necessary to
transmit and receive signals according to IEEE 802.15.4 or proprietary ISM data rates.
The balun B1 performs the differential to single-ended conversion of the RF signal to connect the AT86RF212B
to the SMA connector X2.
