MT312 Tuner Control
4 Tuner Control
4.1 Simple Channel Change Sequence
Program tuner via GPP in 'pass through mode'
If the MT312 is running, to change channel keeping
the same signal conditions, it is only necessary to
change the tuner data and possibly the DiSEqC™
data. NO reset is necessary.
open port with Reg 20 = 64 (40hex)
send TUNER DATA via I2C bus (5 bytes).
close port with Reg 20 = 0
DiSEqC mode
4.2 Channel Change Sequence with a new
Symbol Rate
eg Vertical with 22kHz on:
Reg 22 = 1 (01hex)
If the MT312 is running, to change channel and
Symbol rate but not Viterbi coding rate, change the
tuner data and possibly the DiSEqC™ data and
Symbol rate. NO reset is necessary.
GO
Re-acquire signal
Reg 27 = 1
4.3 Channel Change Sequence with Search
Mode
Figure 16 - Simple channel change sequence
If the signal parameters are unknown, it is possible to
instruct the MT312 to find a digital signal and report
the parameters found. Registers 24 and 25 are
programmed with the expected range(s) and the
search mode bit SYM RATE[B15] is set high. A code
rate search is forced by programming more than one
bit in VIT MODE (26) register. The IQ spectrum
phase can be automatically determined by setting bit
7 in the VIT MODE (26) register.
Program tuner via GPP in 'pass through mode'
open port with Reg 20 = 64 (40hex)
send TUNER DATA via I2C bus (5 bytes).
close port with Reg 20 = 0
DiSEqC mode
Note: code rate 6/7 is not searched for DVB mode.
eg Horizontal with 22kHz on:
Reg 22 = 65 (41hex)
If a signal with the specified symbol rate range (or
ranges) is not found in the frequency range
searched, a QPSK Baud End interrupt (Bit B6, QPSK
INT L (2)) is issued.
Signal input - Symbol rate
eg 22.0 MBaud:
Reg 23 = 22 (16hex)
Reg 24 = 0
When the MT312 QPSK section has locked to the
signal, this is indicated in register (6) by QPSK STAT
H[B0] = 1. The symbol rate found can be read from
registers (123 - 124) MONITOR, provided the
register (103) MON CTRL = 3. The tolerance of the
result is ±0.25%. The 14 MSBs of this result
(discarding two LSBs) may be written as the 14 LSBs
of the 16-bit register pair (23 and 24) SYM RATE in
the non-search mode for re-acquisition of the same
channel.
Viterbi code rate
eg V_IQ swap not set, CR = 5/6:
Reg 25 = 8 (8hex)
GO
Re-acquire signal
Reg 27 = 1
The FEC is locked to the signal, when the Byte Align
lock in FEC STATUS[B2] = 1. Then the code rate
found can be read from FEC STATUS[B6-4], see
register 6 49 for details.
Figure 17 - Channel change sequence with new
Symbol rate, DVB mode
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