Hardware
         

This project is based on PIC24EP512GP202 microcontroller, which is 16-bit Microchip's MCU with 512K of internal flash program memory, 48K of internal data memory, packed in standard 28-pin Shrink-DIP case.

The hole unit uses +5V power supply (2.1 mm coaxial CON1A or mini USB CON1B connector, but take care not to use both!). Measured current consumption is 77 mA. LDO regulator MCP1702-33 is used for +3.3V supplu for MCU. Some pins of this MCU are 5V tolerant, you can check the details in the datasheet, available here.

Instead of quarz, you can use ceramic resonator for the oscillator, but some of resonators will produce significant frequency jitter, which is visible as horizontal pixel instability on the screen. It is also possible to use internal FRC oscillator with PLL, but, of course, the jitter will be even worse.

R, G and B video hardware drivers are realized by simple passive attenuators (R1R2, R4R5 and R7R8) and NPN transistors (as emitter followers) T1-T3. There are no special requirements for those transistors, but the low current types (=< 100 mA) are reccomended, as the high current ones typically have lower bandwidth. Intensity control (for "dark" colours and gray) uses standard silicon diodes D1-D4. Do not use Schottky diodes, as their forward voltage drop is too low for proper dark colour representation in analog video signal.

5-channel audio mixer uses simple passive resistor network. Capacitor C9 is used for passive, first order low-pass RC filter. Pull-up resistor R14 is used in two-step volume level control circuit. As all outputs are pure binary (on-off), low volume level is managed by software, when the output port pins are held in open-drain state (selectable in ODCA and ODCB registers), as in that case resistors R14-R20 create additional voltage attenuator, for each output individually.

 


Schematic diagram v2


All components, including keys and connectors, are soldered on 2-layer PCB, dimensions 50x110 mm. Here is the screenshot of the PCB (green = bottom layer, red = top layer).

 

PCB v2

 

 Note that PCB v2 drawing is not to the photo on the intro page, e.g. arrow keys are turned diagonaly to enable closer key positions. The photo is made with the current (v1) vesion, and PCB drawing and the schematic diagram on the top of this page is new (v2) version. New PCB is ordered and will be finished at the end of January, then I shall update the project. I hope that there are no errors on the PCB v2 drawing, but it will be tested only when I get the PCBs. Anyhow, you can download PCB file (RGC3.PCB, in Protel PCB format) here, it is zipped together with software. 

Here follows the component placement on the PCB, mechanical details and bill of material.

Bill Of Material


ICs:

IC1     PIC24EP512GP202-I/SP
IC2     MCP1702-33

Transistors:
T1    BC547
T2    BC547
T3    BC547
T4    BC547

Diodes:
D1    1N4148
D2    1N4148
D3    1N4148
D4    1N4148
D5    1N4148
D6    LED 3mm

Quarz:
Q1    8MHz

Resistors (raster 7.5 mm):
R1     1K5
R2     1K5
R3     51Ω
R4     1K5
R5     1K5
R6     51Ω
R7     1K5
R8     1K5
R9     51Ω
R10   1K5
R11   120Ω
R12   1K5
R13   120Ω
R14   1K5
R15   5K6
R16   5K6
R17   5K6
R18   5K6
R19   5K6
R20   1K5
R21   10K
R22   10K
R23   5K6
R24   5K6
R25   10K
R26   1M
R27   2K2

Capacitors:
C1     100nF ceramic
C2     4.7μF tantalum
C3     4.7μF tantalum
C4     100nF ceramic
C5     100 nF ceramic
C6     4.7μF tantalum
C7     27pF ceramic
C8     27pF ceramic
C9     100nF ceramic

Connectors:
Con 1A    Coaxial 2.1mm for PCB
Con 1B    USB Mini-A for PCB
Con 2      15-pin VGA  for PCB
Con 3      Coaxial 3.5mm stereo for PCB
Con 4      Nylon 5-pin raster 2.54mm 90
Con 5      Nylon 2-pin raster 2.54mm 90

Mechanical components:
Please see drawing

 

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