Rainbow Electronics AT86RF231 - Manual (Page 125)

125
8111A–AVR–05/08
AT86RF231
9.8 Automatic Filter Tuning (FTN)
9.8.1 Overview
The FTN is incorporated to compensate device tolerances for temperature, supply voltage varia-
tions as well as part-to-part variations of the radio transceiver. The filter-tuning result is used to
correct the analog baseband filter transfer function and the PLL loop-filter time constant, refer to
Section 4. “General Circuit Description” on page 10 .
An FTN calibration cycle is initiated automatically when entering the TRX_OFF state from the
SLEEP, RESET or P_ON states.
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. If necessary, a
calibration cycle is to be initiated in states TRX_OFF, PLL_ON or RX_ON. This applies in partic-
ular for the High Data Rate Modes with a much higher sensitivity against BPF transfer function
variations. The recommended calibration interval is 5 min or less.
9.8.2 Register Description
Register 0x18 (FTN_CTRL):
This register controls the operation of the filter tuning network calibration loop.
• Bit 7 - FTN_START
FTN_START = 1 initiates the filter tuning network calibration. When the calibration cycle has fin-
ished after at most 25 µs the register bit is automatically reset to 0.
• Bit [6:0] - Reserved
Bit 7 6 5 4 3 2 1 0
+0x18 FTN_START Reserved FTN_CTRL
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Reset Value 0 1 0 1 1 0 0 0