L64777 A LSI Logic Corporation, L64777 A Datasheet - Page 15

• Manufacturer Part Number: L64777 A
• Description: Communications, DVB QAM Modulator
• Manufacturer: LSI Logic Corporation

Global Control and PLL Module
Interpolator
Serial Microprocessor Interface
module generates the desired pulse shape by combining the outputs of
four identical filter branches for I and Q.
For an oversampling factor of four, the filter executes the above
sequence at four times the symbol rate (60 MHz in PLL mode).
The L64777 interface supports serial and parallel input modes at the
input interface. The global control generates the clocking for the input
and output interfaces; it also controls the data path. It contains all the
necessary logic to chain the processing units together.
The global control manages the output data stream so that it is
continuous (no gaps between the symbols), assuming that the incoming
data rate is constant (on average). To achieve this, a PLL must derive
the output clock OCLK from the input transport stream rate.
The PLL module consists of two independent clock dividers for ICLK and
OCLK. The dividers are 15-bit binary counters that have a count
sequence length that is programmable.
The default values written by the external microprocessor are: OCLK
divider = 32, ICLK divider = 6 for 64 QAM.
In PLL Mode 2, the interpolator retimes the output samples that the
Nyquist filter calculated. The interpolator is clocked with PCLK and
generates the output samples in the PCLK sampling grid. The
interpolator takes the required retiming information from the NCO.
PCLK is at least twice the frequency of the original OCLK obtained by
the formula for PLL Modes 1 and 2. The square root raised cosine filter
also compensates for the sin(x)/x frequency characteristic of the
digital-to-analog converter with the faster sampling grid.
The external microprocessor controls the QAM mode of operation, 16 to
256 QAM. It also controls the mode of input synchronization (that is,
whether to lock synchronization to sync bytes or input pulses). The
L64777 DVB QAM Modulator
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