15
6.2 Control
Detailed control of the tuner is implemented by the ST72324 (U7001 [sheet 3, C6]) micropro-
cessor upon instructions from the CS98200 SPI bus on the main board. U7001 is responsible
for:
•
Handling all control commands from the main board.
•
Communicating with the PLL IC (U2074 [sheet 2, C/D3]) via the CCB bus to set it’s various
•
I/O pins as well as setting the desired local oscillator frequency.
•
Querying the RDS chip (U2200 [sheet 2, D6]), if present, for relevant messages.
•
Processing the seek algorithms based on S-Meter and audio levels, as well as IF-count.
•
Storing the seek/stop and stereo levels during tuner alignment for use in the field.
The general purpose input and output ports on the PLL IC (U2074) are set up as follows:
THEORY OF OPERATION
U2074 Pin
Number
Name Direction
Function/Notes
7
FM/AM
Output
Controls Q2000 which switches power to the FM
front-end and the IF amplifier. In AM mode these
are both switched off.
8
IF/MUTE
O.C. Output Enables the audio output of the detector when low.
See pin 13.
9
AM/FM
Output
Switches the mode of detector IC U2000 and sets
which output (FM : pin 23, AM : pin 24) is active.
Inactive => high impedance.
10 FORCE-
MONO
Output
Forces the detector IC to decode FM into mono-
aural audio.
11 Supply_Freq_
Sel
Output
See above table
13
IF/MUTE
Input
When pin 8 is high impedance R2013 pulls the DC
level of the IF/MUTE line high (>3.5v), audio is
muted and the output of the FM IF buffer appears
on this line. This is fed to the IF counter on pin 13.
This is used during seek to determine if a valid
broadcast signal is present. Note that this output is
dependent on having TU-LED (Pin 6) low indicating
a sufficient input IF level.
14
ST-LED
Input
Used to monitor the stereo indicator coming from
pin 7 of U2000. The state of this pin is shown on
the stereo icon display or in the OSD FM status
window