bt8970 Mindspeed Technologies, bt8970 Datasheet - Page 25

• Manufacturer Part Number: bt8970
• Description: Single-chip Hdsl Transceiver
• Manufacturer: Mindspeed Technologies

Bt8970
Single-Chip HDSL Transceiver
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 level meter provides the monitoring needed for adaptive offset compensation. The offset-adjusted input
signal is accumulated over the meter timer interval [meter_low, meter_high; 0x18, 0x19]. The 16 MSBs are
placed into the DC Level Meter Registers [dc_meter_low, dc_meter_high; 0x44, 0x45].
2.2.3.4 Signal Level Meter
The signal level meter provides the monitoring needed for adjusting the analog gain circuit located prior 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 interface, if enabled. The threshold is determined by the value in the Far-End High Alarm
Threshold Registers [far_end_high_alarm_th_low, far_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].
[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
2.2.4.1 Linear Echo Canceller (LEC)
The Linear Echo Canceller (LEC) is a conventional LMS, 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.
that may be present.
coefficient updates only. A special mode exists to zero all of the coefficients; it is also enabled through the
microcomputer interface.
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.
100101B
The interrupts high_felm and low_felm, are bits 2 and 1, respectively of the IRQ Source Register
When enabled, the last data tap of the echo canceller is treated specially. This serves to cancel any DC offset
A freeze coefficient mode may be specified via the microcomputer interface. This mode disables the
An additional mode exists to zero the output of the FIR with no effect on the coefficients; it is also enabled
Preliminary Information/Conexant Proprietary and Confidential
Conexant
2.0 Functional Description
2.2 Receive Section
Figure
2-3.
2-7