Rainbow Electronics ATmega128RFA1 - Manual (Page 83)

83
8266A-MCU Wireless-12/09
ATmega128RFA1
9.6.6.4 Calibration Loops
Due to temperature, supply voltage and part-to-part variations of the radio transceiver
the VCO characteristics diverge. Two automated control loops are implemented to
ensure a stable operation: center frequency (CF) tuning and delay cell (DCU)
calibration. Both calibration loops are initiated automatically when the PLL is enabled
during state transition from TRX_OFF to PLL_ON. The center frequency calibration is
additionally initiated when the PLL changes to a center frequency of another channel.
It is recommended to initiate the calibration loops manually if the PLL operates for a
long time on the same channel e.g. more than 5 min or the operating temperature
changes significantly. Both calibration loops can be initiated manually by setting
PLL_CF_START = 1 of register PLL_CF and PLL_DCU_START = 1 of register
PLL_DCU. The device must be in PLL_ON or RX_ON state to start the calibration. The
completion of the center frequency tuning is indicated by a TRX24_PLL_LOCK
interrupt.
Both calibration loops may be run simultaneously.
9.6.6.5 Interrupt Handling
Two different interrupts indicate the PLL status. The TRX24_PLL_LOCK interrupt
indicates that the PLL has locked. The TRX24_PLL_UNLOCK interrupt indicates an
unexpected unlock condition.
A TRX24_PLL_LOCK interrupt is supposed to occur in the following situations:
• State change from TRX_OFF to PLL_ON / RX_ON/ RX_AACK_ON/
TX_ARET_ON;
• Channel change in states PLL_ON / RX_ON/ RX_AACK_ON/ TX_ARET_ON;
Any other occurrences of PLL interrupts indicate erroneous behavior and require
checking of the actual device status.
The state transition from BUSY_TX to PLL_ON after successful transmission does not
generate a TRX24_PLL_LOCK interrupt within the settling period.
9.6.7 Automatic Filter Tuning (FTN)
The FTN is incorporated to compensate device tolerances for temperature, supply
voltage variations as well as part-to-part variations of the radio transceiver. The filter-
tuning result is used to correct the transfer function of the analog baseband filter and
the time constant of the PLL loop-filter (refer to
"General Circuit Description" on page
30 ).
An FTN calibration cycle is initiated automatically when entering the radio transceiver
TRX_OFF state from the SLEEP or RESET state.
Although receiver and transmitter are very robust against these variations, it is
recommended to initiate the FTN manually if the radio transceiver does not use the
SLEEP state. A calibration cycle is to be initiated in states TRX_OFF, PLL_ON or
RX_ON if necessary. This applies in particular to the High Data Rate Modes with a
much higher sensitivity to variations of the BPF transfer function. The recommended
calibration interval is 5 min or less.
9.7 Radio Transceiver Usage
This section describes the basic procedures to receive and transmit frames with the
radio transceiver.