VP310

PRELIMINARY DATA

12

1.5 Control

Automatic Symbol Rate Search, Code Rate Search, Signal Acquisition and Signal Tracking

algorithms are built into the VP310 using a sophisticated on-chip controller. The software

interaction with the device is via a simple Command Driven Control (CDC) interface. This CDC

maps high level inputs such as symbol rates in MBaud and frequencies in MHz, to low level on-

chip register settings. The on-chip control state machine and the CDC significantly reduces the

software overhead as well as the channel search times. There is also an option for the host

processor to by-pass both the CDC as well as the on-chip controller and take direct control of the

QPSK demodulator.

Command

Driven

Control

High level input/output

(MBaud, MHz)

VP310

format

registers

Acquistion/

Track

State machine

QPSK

Low level register read/write

Figure 10. VP310 Control Structure.

Once the VP310 has locked up, any frequency offset can be read from the LNB_FREQ error

registers 7 and 8. The frequency synthesiser under the software control can be re-tuned in

frequency to optimise the received signal within the SAW bandwidth. Note that VP310

compensates for any frequency offsets before QPSK demodulation. Hence a frequency offset will

not necessarily lead to a performance loss. Performance loss will occur only if part of the signal

is cut off by the SAW or base-band filter, due to this frequency offset. This will happen only if the

symbol rate is close to maximum supported by that filter. In such an event it is recommended

that front-end be re-tuned to neutralise this error before the SAW filter. It is then necessary for the

VP310 to re-acquire the signal.

The VP310 can generate control signals to enable full control of the dish and LNB. The chip

implements the signals needed for the full DiSEqC v1.1 specification. This includes high/low

band selection, polarisation and dish position.

The microprocessor interface is via the primary I²C bus. The tuner control from the VP310 is via

either I²C bus or 3-wire bus, recreated on the General Purpose Port (GPP).

1.5.1 Known Symbol Rate and Code Rate mode

In this mode, the Symbol rate in MBaud and Viterbi code rate are the only values needed to start

the VP310 searching for the signal. The CDC module maps the high level parameters into the

various low level register settings needed to acquire and track the signal. The low level registers

may be read and directly modified to suit very specific requirements. However, this is not

recommended.