Processor Board theory
of design and operation
The major blocks of the processor board are the CPU itself (a PIC18F2620 28 pin DIP microcontroller running on its internal resonator at 8 MHz), a MAX232 RS-232 level converter and DB9 connector to allow in-circuit programming via a boot loader, a set of 3 analog-to-digital converters and associated circuitry for inputs, and a set of 3 high current relay drivers as outputs. The block diagram is as follows:
The Processor:
In the modern age, high performance microcontrollers are
both extremely capable and extremely inexpensive. For this project, the 18F2620 PIC processor
was chosen for its large internal flash RAM program storage, high performance,
and outstanding flexibility.
The power supply:
The processor board includes a very standard 7805 voltage regulator based power supply, capable of delivering up to 1 amp at +5 volts. There is a 3.5mm power jack that can be connected to a +8 - +15 volt supply. Between the jack and the 7805 is a 10 uF electrolytic capacitor to provide surge capacity, and after the 7805 is a .01uF bypass cap for RF filtering.
The programming
circuit:
This circuit is only for use when the PIC microcontroller needs to be reprogrammed. It serves no purpose when the system is deployed and in use, other than to allow “in the field” software upgrades. The processor is loaded with the Tiny Bootloader. This bootloader allows software on the PC to download new firmware into the PIC’s flash ram via a serial cable. The MAX232 converts the RS-232 protocol’s -12 volt to +12 volt signals into the 0 volt to +5 volt signals that the processor expects. The reset circuit allows the PC to assert a signal on pin 7 of the DB9 connector (usually used for RTS), which turns on a 2n2222 transistor which in turn grounds the MCLR pin on the processor, resetting it. MCLR is otherwise pulled high with a 10k resistor to VCC.
The Input circuit:
The goal was to provide a large number of general purpose voltage level inputs that could be used for various measurement and instrumentation applications. Examples might include the monitoring of power supply voltage or (using a shunt) current, measurement of temperature using an LM34 type voltage output temperature sensor, or measurement of forward or reverse RF power using a voltage output directional coupler or bridge such as the Wavenode HF-1.
The board provides 24 general purpose inputs, four of which are intended to be used to attach one or two Wavenode RF directional couplers. A companion board which includes appropriate female PCB mounted 6 pin minidin connectors, 4:1 voltage dividers, and RF bypassing capacitors, also provides a mating 5 pad connection so that it can easily be connected to the processor board with either a ribbon cable or (if the boards are fabricated on a single piece of FR4 and not separated) with 5 short jumpers.
All 24 inputs are bypassed with .01uF capacitors to avoid any stray RF getting into the analog circuitry, and buffered with TLC2274 rail to rail op amps (the same ones that Wavenode uses in their input circuitry).
Each set of 8 inputs are fed into an ADC78H90 analog to digital converter, and each converter is provided with a 4.096 volt reference generated by an LM4040-4.1 shunt regulator and conditioned by a capacitor network as suggested in the 78H90 datasheet. Since these converters are 12 bit devices, each output bit represents 1 millivolt for convenience. The 78H90’s are interfaced to the PIC using the processor’s built in SPI module.
Care was taken in the layout of the 78H90 section of the board to avoid digital signals passing under the parts, and to ensure a low impedance “star” single point connected ground configuration.
The Frequency
Counter:
In addition to the general purpose analog-to-digital inputs, one special purpose input is provided for a frequency counter to determine current operation frequency. Circuitry is provided on a companion PCB to condition an input signal, this circuit was based on that in the NorCal QRP FCC-1 kit, and is sensitive to signals as low as 30 mV rms. On the processor board itself, there is a pad provided that mates to the output pad of the companion board (or any other suitable frequency input), and connects to the CCP1 pin on the 18F2620. Using the on-board 1:256 pre-scaler, this allows the ‘2620 to measure HF frequencies with ease.
The output
circuit:
The goal was to provide a large number of general purpose high current outputs, that could be used to drive LED’s, relays, or other devices. Since it was intended that this processor board be used for a variety of applications, including at a minimum control of a solid state amplifier (which would require the control of T/R switching relays as well as output filter selection relays and front panel indicator LED’s) and control of a companion antenna tuner (which would require the control of a large number of inductor and capacitor selection relays), 24 outputs are provided.
These outputs are current sinking, i.e. when activated they provide a connection to ground. This is accomplished by TPIC6B595 shift register relay drivers, which are capable of sinking 150 MA at up to 50 volts on each of their 8 outputs simultaneously at 25 degrees C.
The ‘6B595’s are interfaced to the processor using a simple 4 wire serial interface, and are cascaded so that the processor can reset all 24 outputs by simply shifting out 3 bytes.
The PWM output
In addition to the general purpose outputs, one special purpose output is provided for pulse-width modulation control of fans or other DC devices. The ‘2620 processor has sophisticated PWM circuitry built in. A companion PCB provides an IRL520 logic level MOSFET capable of controlling up to 10 amps of current at up to 100 volts. This is more than enough to allow for temperature based speed control of cooling fans in amplifier applications, since even large 100 CFM class fans generally draw less than 10 watts each.