Rockwell SoniCrafter BT8960 - Manual (Page 36)

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2.0 Functional Description
2.2 Receive Section
Bt8960
Single-Chip 2B1Q Transceiver
N8960DSB
The LFSR operates in the same way in both cases, except in the two-level case
it is clocked once-per-symbol and in the four-level case it is clocked twice-per-
symbol.
When operating as a scrambler, the LFSR must first be locked to the far-end
source. Once locked, it is then able to replicate the far-end input sequence, when
its input is held at all ones. The locking sequence is controlled internally, initiated
through the microcomputer interface by setting the lfsr_lock bit of the
detector_modes register. The locking sequence consists of the following four
steps:
1. Operate the LFSR as a descrambler for 23 bits.
2. Operate the LFSR as a scrambler for 127 bits. The sync detector is active
during this period.
3. Go to Step 1 if synchronization was not achieved, otherwise continue to
Step 4.
4. Send an interrupt to the microcomputer, if unmasked, indicating successful
locking and continue operating as a scrambler.
The sequence continues until the lfsr_lock control bit is cleared by the micro-
computer.
2.2.6.5 Sync Detector The sync detector compares the output of the scrambler with the output of the
symbol detector. The number of equivalent bits is accumulated for 128 compari-
sons. The result is then compared to a Scrambler Synchronization Threshold Reg-
ister [scr_sync_th; 0x2E], lock is declared, and the sync bit of the irq_source
register is set if the count is greater than the threshold. For a count less than or
equal to the threshold, no lock condition is declared and the sync bit is unaffected.
2.2.6.6 Detector Meters The detector consists of five meters: a BER meter, a symbol histogrammer, a
noise-level meter, a noise-level histogram meter, and an SNR alarm meter.
The BER meter provides an estimate of the bit error rate when the received
symbols are known to be scrambled ones. When the LFSR is operating as a
descrambler the meter counts the number of ones on the descrambler output.
When the LFSR is operating as a scrambler, the BER meter counts the number of
equal scrambler, and symbol detector outputs. The counter operates over the
meter timer interval [meter_low, meter_high; 0x18, 0x19]. The counter is satu-
rated to 16 bits. At the end of the measurement interval the counter is loaded into
the Bit Error Rate Meter Registers [ber_meter_low, ber_meter_high; 0x4C,
0x40].
The symbol histogrammer computes a coarse histogram of the received sym-
bols. It operates by counting the number of ones received during meter timer
interval [meter_low, meter_high; 0x18, 0x19]. That is, at the start of the measure-
ment interval a counter is cleared. For each detector output which is +1 or –1, the
counter is incremented. If the detector output is +3 or –3, the count is held at its
previous value. The count is saturated to 16 bits. At the end of the measurement
interval, the 8 MSBs of the counter are loaded into the Symbol Histogram Meter
Register [symbol_histogram; 0x4E].
The noise level meter estimates the noise at the input to the slicer. It operates
by accumulating the absolute value of the slicer error over meter timer interval
[meter_low, meter_high; 0x18, 0x19]. At the end of the measurement interval, the
16 MSBs of the 32-bit accumulator are loaded into the Noise Level Histogram
Meter Register [nlm_low, nlm_high; 0x50, 0x51].