Le58083
Data Sheet
internal signal processing path of the device. Therefore, accuracy is not limited to the weight of the LSB, and a weight of half this
value is realizable.
When this robbed-bit mode is selected, not every frame contains bits for signaling, and therefore not every byte requires its LSB
substituted with the half-LSB weight. This substitution only occurs for valid PCM time slots within frames for which this robbed bit
has been designated. To determine which time slots are affected, the device monitors the frame sync (FS) pulse. The current
frame is a robbed-bit frame and this half-LSB value is used only when this criteria is met:
■ The RBE bit is set, and
■ The device is in the µ-law companding mode, and
■ The current frame sync pulse (FS) is two PCLK cycles long, and
■ The previous frame sync pulse (FS) was not two PCLK cycles long.
The frame sync pulse is sampled on the falling edge of PCLK. As shown in Figure 24, if the above criteria is met, and if FS is
high for two consecutive falling edges of PCLK then low for the third falling edge, it is considered a robbed-bit frame. Otherwise,
it is a normal frame.
Figure 24. Robbed-Bit Frame
PCLK
FS
Normal Frame (Not Robbed-Bit)
PCLK
FS
Robbed-Bit Frame
Default Filter Coefficients
The Le58083 Octal SLAC device contains an internal set of default coefficients for the programmable filters. The default filter
gains are calculated based on the application circuit shown on page 91. This SLIC device has a transmit gain of 0.5 (GTX) and
a current gain of 500 (K1). The transmit relative level is set to +0.28 dBr, and the receive relative level is set to –4.39 dBr. The
equalization filters (X and R) are not optimized and the Z and B filters are set to zero. The nominal input impedance was set to
812 Ω. If the SLIC device circuit differs significantly from this design, the default gains cannot be used and must be replaced by
programmed coefficients. The balance filter (B) must always be programmed to an appropriate value.
To obtain this above-system response, the default filter coefficients are set to produce these values:
GX gain = +6 dB, GR gain = –8.984 dB
AX gain = 0 dB, AR gain = 0 dB, input attenuator on (DGIN = 0)
R filter: H(z) = 1, X filter: H(z) = 1
Z filter: H(z) = 0
B filter: H(z) = 0
AISN = cutoff
Notice that these default coefficient values are retained in read-only memory areas within the Le58083 Octal SLAC device, and
those values cannot be read back using any data commands. When the device is selected to use default coefficients, it obtains
those values directly from the read-only memory area, where the coefficient read operations access the programmable random
access data memory only. If an attempt is made to read back any filter values without those values first being written with known
programmed data, the values read back are totally random and do not represent the default or any other values.
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Zarlink Semiconductor Inc.
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