Comparing PSoCs and MCUs in digital TV design

Ronak Desai, Cypress Semiconductor

July 16, 2012

Ronak Desai, Cypress Semiconductor

When the user selects the channel search function, the microcontroller tunes the local oscillator for respective frequencies. The user can fine-tune any channel using the remote control by adjusting the local oscillator frequency. The TV tuner has a step size of 31.25KHz, 62.5KHz, 125KHz, etc. The microcontroller can increase or decrease the frequency of the local oscillator by adjusting the tuner step size based on the fine tuning selection by the user.

Audio Output Block: The microcontroller controls the volume of the speaker through an external 8-bit/12-bit DAC (Digital to Analog converter). It will increase, decrease or mute the volume of the speaker through an external DAC and audio amplifier.

Power Supply Block: The digital television uses an SMPS (Switch mode power supply) to convert the input AC voltage to a DC output. It also uses an on-board LDO (Low dropout regulators) to step down DC voltages.

Using an MCU to controller the tuner’s PLL
Every tuner has an internal PLL (phase locked loop) block (Figure 3 below). The function of this PLL is to provide the tuning voltage (Vtune), which is a fixed DC voltage. The tuning voltage will vary from channel to channel. The PLL block receives the local oscillator frequency at the input, which will be amplified by an internal pre-amplifier. A pre-scaler will downconvert the input frequency to ÷ 64, ÷ 128, ÷ 512, ÷ 1024, etc.

Click on image to enlarge.

Figure 3 Tuner and PLL Block
Microcontroller data through the I2C goes to the programmable divider, which also receives the input from the reference oscillator (for example, a 4 MHz crystal oscillator inside the tuner block).

The phase comparator (i.e., phase detector) receives an input from the local oscillator frequency (for example 87.15MHz) through the pre-scaler and another input from the microcontroller (for example, 87.15MHz) through the reference divider and reference oscillator. If both inputs match, the phase comparator will provide the tuning voltage which is Vtune. Any mismatch between the local oscillator frequency and frequency data from the microcontroller will cause no tuning voltage (Vtune) at the tuner output and also no IF output.

The PLL creates a closed loop by tuning the local oscillator frequency with the help of te microcontroller data and generates the tuning voltage at the output. The tuning voltage will increase from lower frequency channels to higher frequency channels.

The microcontroller adjusts the step size of the tuner for fine tuning by varying the pre-scaler and programmable divider values. Step Size = (Local Oscillator Frequency/ Pre-scaler) X (Programmable divider/ Reference oscillator). Some possible configurations are shown below:

Configuration #1:
Local Oscillator Frequency: 87.15MHz
Pre-scaler: ÷ 64
Programmable divider: 256
Reference oscillator: 4MHz
(87.15MHz / 64) X (256/ 4MHz) = 87.15MHz (No Step size)

Configuration #2:
Local Oscillator Frequency: 87.15MHz
Pre-scaler: ÷ 1024
Programmable divider: 256
Reference oscillator: 4MHz
(87.15MHz/1024) X (256/4MHz) = 87.15MHz X 0.0625 (62.5KHz Step size)

Configuration #3:
Local Oscillator Frequency: 87.15MHz
Pre-scaler: ÷ 512
Programmable divider: 256
Reference oscillator: 4MHz
(87.15MHz/512) X (256/ 4MHz) = 87.15MHz X 0.125 (125KHz Step size)