2.3.4 Serial Microprocessor Interface
A bidirectional microprocessor interface allows write and confidence
read-back of internal registers. No interaction during operation is required
with the microprocessor, but all registers must be configured after a
RESET to guarantee proper operation of the device. A default setup that
requires no microprocessor download is built in for 64 QAM.
In the L64777, the Group 2 register 0 acts as a sequential download
register that feeds the 196 bytes of filter coefficients. After every write,
the user can read back the last written coefficient to verify the tail entry
of the coefficients shift register. The L64777 uses all other registers
nonsequentially; these can be read back directly.
2.4 Input Synchronization
The L64777 transport interface reads the data stream from the transport
source, identifies the position of the synchronization bytes, and strips off
invalid data. The transport interface can operate in either Parallel or
Serial mode.
The L64777 can synchronize the transport interface in two ways. In both
modes, it works synchronously with ICLK and reads all signals, including
input data, on the raising edge of ICLK.
•
In external synchronization mode, the transport interface specifies
the position of the external sync byte by asserting FSTARTIN HIGH
during sync byte input. In serial mode, the interface must assert the
signal HIGH during the first bit (MSB) of the input stream.
•
In internal synchronization mode, the L64777 does not require a
block start indication and finds the position of the programmed sync
byte automatically.
The transport interface also can apply the signal DVALIDIN, indicating
valid input data, to allow gaps between input bytes. To avoid cyclic buffer
overrun or underrun, the average data input rate, measured over the
programmed block length, must not differ from the nominal payload rate
of the generated QAM signal. The circular buffer inside the L64777
allows a maximum 64-byte compensation of the input stream.
2-10
Modulator Architecture
ETC [ ETC ]
