Preliminary Review
Bt8970 2.0 Functional Description
Single-Chip HDSL Transceiver 2.2 Receive Section
100101B Conexant 2-7
Preliminary Information/Conexant Proprietary and Confidential
11/29/00 10:00 A.M.
2.2.3.2 Offset Adjustment
A nonzero DC level on the input can be corrected by a DC offset value [dc_offset_low, dc_offset_high; 0x26,
0x27] which is subtracted from the input. The DC offset is a 16-bit number and is programmed via the
microcomputer interface.
2.2.3.3 DC Level Meter
The DC le v el meter provides the monito ring needed f or adaptive offset compensati on. The of f set-adj usted i nput
signal is accumulated over the meter timer interval [meter_low, meter_high; 0x18, 0x19]. The 16 MSBs are
placed into the DC Level Meter Regis ters [dc_meter_low, dc_meter_high; 0x44, 0x45].
2.2.3.4 Signal Level Meter
The sign al leve l meter provides the monito ring need e d for adjusting the analog gain circuit l ocated prio r to the
ADC. This value is accumulated over the meter timer interval [meter_low, meter_high; 0x18, 0x19]. The 16
MSBs are placed in the Signal Level Meter Registers [slm_low, slm_high; 1; 0x46, 0x47].
2.2.3.5 Overflow Detection and Monitoring
The overflow sensor detects ADC overflows. The overflow monitor counts the number of overflows, as
indicated by the overflow sensor during the meter timer interval [meter_low, meter_high; 0x18, 0x19]. The
counter is limited to 8 bits. In the case of 256 or more overflows during the measurement interval, the counter
will hold at 255. The counter is loaded into the Overflow Meter Register [overflow_meter; 0x42] at the end of
each measurement interval.
2.2.3.6 Far-End Level Meter
The far-end level meter monitors the output of the echo canceller. Since the echo canceller output had the echo
of the transmitted signal subtracted from it, it is called the far-end signal. This value is accumulated over the
meter timer interval [meter_low, meter_high; 0x18, 0x19]. The 16 MSBs are placed into the Far-End Level
Meter Register [felm_low, felm_high; 0x48, 0x49].
2.2.3.7 Far-End Level Alarm
The result of the far-end level meter is compared to two thresholds. When exceeded, an interrupt is sent to the
microcomputer inter face, if enabled. The threshold is determined by the value in the Far-End High Alarm
Threshold Registers [far_end_high_alarm_th_low, f ar_end_high_alarm_th_high; 0x30, 0x31] and the Far-End
Low Alarm Threshold Registers [far_end_low_alarm_th_low, far_end_low_alarm_th_high; 0x32, 0x33].
The interrupts high_felm and low_felm, are bits 2 and 1, respectively of the IRQ Source Register
[irq_source; 0x05]. The interrupts high_felm and low_felm, can be masked by writing a one to bits 2 and 1,
respectively of the Interrupt Mask Register High [mask_high_reg; 0x03].
2.2.4 Echo Canceller
The Echo Canceller (EC) removes images of the transmitted symbols from the received signal and consists of
two blocks: a linear and nonlinear echo canceller. The organization of the blocks is displayed in Figure 2-3.
2.2.4.1 Linear Echo Canceller (LEC)
The Line ar Echo Cance ller (LE C) is a convent i onal LM S, Finite Impulse Response (FIR) filter, which removes
linear images of the transmitted symbols from the received signal. It consists of a 60-tap FIR filter with 32-bit
linear adapted coefficients.
When enabled, the last da ta tap of t he echo can cel ler is t reat ed speci ally. This serves to can cel any DC of fset
that may be present.
A freeze coefficient mode may be specified via the microcomputer interface. This mode disables the
coefficient updates only. A special mode exists to zero all of the coefficients; it is also enabled through the
microcomputer interface.
An additional mode exists to zero the output of the FIR with no effect on the coefficients; it is also enabled
through the microcomputer interface. Individual EC coefficients can be read and written through the
microcomputer interface. Adaptation should be frozen prior to reading or writing coefficients.