High-End-Preamp （3）

Published:2011/7/22 3:08:00 Author:Phyllis From:SeekIC

By Benjamin Hinrichs

The control centre

The main circuit board, which forms the control centre for the preamplifier, requires surprisingly few components. This is due to the high integration density of the two ICs used here. By far the majority of the components are used to generate clean supply voltages.

The circuit is split into an analogue portion and a digital portion. The digital portion contains a Microchip PIC18LF452 microcontroller clocked at 10 MHz by a crystal oscillator. This microcontroller has 8-bit registers and 16-bit instruction words. Microchip has not shown much flair in assigning part numbers to its PIC microcontrollers. For instance, the PIC16F84, PIC16F628, PIC16F877 and PIC12F675 belong to the PIC14 family, while the PIC18LF452 used here belongs to the PIC 16 family.

The PIC18LF452 has a Flash program memory with a capacity of 32 KB (which is adequate for the rather extensive software), 1.5 KB of RAM and a 256-byte EEPROM. Its 31 stack levels provide adequate manoeuvring room for calling functions and procedures if the contents of all of the registers are written to the stack to allow the called procedure to use the registers. When control is returned to the calling procedure, the register contents are retrieved from the stack to allow the calling procedure to continue processing from the point where it transferred control. If frequently used subroutines are implemented using functions and procedures, the resulting interleaving of program execution can quickly exceed the capacity of a relatively shallow stack.

Before discussing the software in any more detail, let’s have a look at the peripheral resources available to the microcontroller. The volume control (IC2) is connected to the microcontroller via the serial SPI bus. In addition, the microcontroller can select the PGA2311 using the CS line, and it can mute the output by placing a Low level on the MUTE line. These four lines, as well as the data output line (SDO), are externally accessible to allow several volume controls to be connected in parallel (as described in Part 2 of this article).

The remainder of Port D and all of Port C are fitted with pull-down resistors (consisting of the two SIL arrays R3 and R4) and routed to pin header K5, to which the pushbutton switches for controlling the preamplifier are connected.

The functions are essentially self-explanatory, but as you might imagine, additional functions are also implemented using combinations of buttons. All of the functions can also be selected using a remote control unit. IC3 is a 36-kHz infrared receiver, which filters, demodulates and cleans up the received light signal and boosts it to TTL levels, all without a single external component. An RC5 decoder is built into the software, so all types of RC5 remote control units (Philips, Grundig, etc.) can be used to control the preamplifier. The IR receiver is connected to RE2, which is one of the three Port E lines.

The configuration options are so manifold that without a clearly organised display presentation you could quickly loose track of where you are, particularly when prograiruning the basic settings. Via Port A, the microcontroller software drives an LC display with two lines of 16 characters and background illumination. In normal operation the display shows the channel names and volume setting, while in Set-up mode it is used to select channel designations and basic volume control settings. Pull-up resistor RIO connected to RA4 is necessary because this port lead has an open-drain output and thus cannot switch to a High level without a pull-up resistor. Trimpot PI adjusts the display contrast. while trimpot P2 adjusts the brightness of the background illumination. JP3 extends the adjustment range. The microcontroller can switch the background illumination on or off via port line REO and transistor Tl.

The microcontroller drives the relay board via K4. Each of port lines RB0-RB7 selects one of the eight audio inputs. The behaviour of the Status LED (Dl) can be configured using the Set-up menu. This is described in more detail later on. along with the significance of the three lines MCLR, RB6 and RB7 that are led out from the board.

The main circuit board has separate power distribution for the analogue and digital portions. The ground potentials must be connected at a suitable location via wire bridge JP2. The single -f 5-V digital supply voltage and the symmetrical ±5-V analogue supply voltages are stabilized in the traditional manner using fixed voltage regulators with the customary buffer and decoupling capacitors. For all three voltages, 5.6-V Zener diodes are provided as ’backup’ safety devices in case excessively high voltages appear on the outputs of the fixed voltage regulators.

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