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1Step 1
The first step is to decide what is included in the controller and for what function.
1. Micro-controller PIC18fF87K22 for flexibility at specification change time.
2. WiFi module from Microchip because the "stack" is already written and tested.
3. Triac outputs for 24VAC operation only will keep it simple.
4. Power Supply from 24VAC and battery backup for Real Time Clock (RTC)retention.
The rest of the elements are support and generally included in any design. Reset circuit, power detection circuit, Triac drivers, etc. Now build your schematic. Experience will give you the correct approach. (It worked before with little or no adjustment.)
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2Step 2
Develop the schematic starting with the micro-controller.
1. Place the selected micro-controller symbol into the schematic. It is suggested that the micro-controller symbol be a semi pictorial viewed from the top with the pin numbers placed at their physical location on the package and associated with an abbreviated function description for that pin. It is convenient to orient pin 1 of the micro-controller on the schematic with the anticipated pin 1 orientation on the printed circuit board (PCB) layout. This makes it easier to visualize the trace routing when laying out the PCB. (For those that do not use an auto router.)
2. Insert those circuits and their symbols that are mandated by the use of the micro-controller. These would be the reset circuit, the RTC external oscillator, the Vdd and Vss connection(s) and their bypass capacitors, the debugging and programming connector and Vcap bypass capacitor. Reserve other ports of the micro-controller as necessary such as the Serial Peripheral Interface (SPI) ports and the Slave Select (SSx) port for the SPI being used. The rest of the ports will essentially be input/output (I/O) for the controller's functions. Since some ports of this device are more suited to input functions and others to output functions a review of the micro-controller's data sheet, particularly port specifications, will assist in the next stage.
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