USB-controlled monochrome LCD display
This is my first stab at driving a graphical display device, and also at USB communication. This project was just meant to be a proof-of-concept, and didn't evolve far off the breadboard.
Design features:
- 128x64 monochrome LCD display with white LED backlight
- USB bus-powered
- Full-speed USB virtual COM port
- Displays graphics sent from a program on the computer
- Manual contrast control
Project status
The project is abandoned in favor of the color LCD display. The circuit works fine on a breadboard, but I messed up the PCB design and didn't want to order a replacement. Also, I didn't like the contrast (or lack thereof) of the FSTN display.
The design posted here is the fixed PCB design.
Bill of materials
| C1 - | PIC24 voltage regulator capacitor: | 4.7µF, tantalum, ESR<2Ω | (3216 metric) | Any appropriate |
| C2,C3 - | Filter capacitors for PIC24: | 0.1µF | (0805) | Any appropriate |
| C4 - | USB voltage regulator capacitor: | 0.33µF | (0805) | Any appropriate |
| C5 - | Filter capacitor for PIC18: | 0.1µF | (0805) | Any appropriate |
| C6 - | Stabilizing capacitor: | 1µF | (0805) | Any appropriate |
| C7,C8 - | Oscillator capacitors: | 22pF | (0805) | Any appropriate |
| J1 - | Header for LCD module: | 20-pin female, 0.1" pitch | (1x20) | Sullins PPPC201LFBN-RC |
| J2,J3 - | Programming headers: | (Not placed - program the microcontrollers by holding a free-hanging male header in these holes) | ||
| J4 - | USB connector: | Female receptacle | (USB Mini B) | Hirose UX60A-MB-5ST |
| Q1 - | Backlight control transistor: | N-channel MOSFET, >5V, >30mA | (SOT23) | ON Semiconductor 2N7002ET1G |
| Q2 - | LCD control transistor: | N-channel MOSFET, >5V, >4mA | (SOT23) | ON Semiconductor 2N7002ET1G |
| R1 - | MCLR pull-up resistor for PIC24: | 10kΩ | (0805) | Any appropriate |
| R2 - | LCD contrast potentiometer: | 10kΩ | Bourns 3352T-1-103LF | |
| R3 - | Current-limiting resistor for backlight: | 47Ω, 1/8W | (0805) | Any appropriate |
| R4 - | MCLR pull-up resistor for PIC18: | 10kΩ | (0805) | Any appropriate |
| U1 - | Primary microcontroller: | 16-bit PIC with 2K RAM | (SOIC-28) | PIC24HJ32GP202 |
| U2 - | USB to serial converter: | 20-pin PIC with USB | (SOIC-20) | PIC18F14K50 |
| U3 - | Voltage regulator for PIC24: | 3.3V output | (SOT23A) | Microchip MCP1700T-3302E/TT |
| X1 - | Oscillator crystal: | 12.000MHz | (HC49/US) | Abracon ABLS-12.000MHZ-B2-T |
| N/A - | LCD module: | 128x64, FSTN, white backlight | Newhaven Display NHD-12864WG-BTFH-V#N | |
| N/A - | Header for LCD module: | 20-pin male, 0.1" pitch | (1x20) | Any appropriate |
Design files
Design details
Power supply
The device is bus-powered, which means it can pull up to 100mA at 5V from the USB power pin. Since start-up capacitance is limited in the USB specification, I don't power up the LCD module or backlight until the microcontrollers have been running for a while.
I use low-side MOSFETs to control the LCD and backlight power. My initial PCB design had them on the high side, but once the transistors were turned on the first time, there was a negligible voltage difference between the drain (+), the gate and the source (–) pins, so it didn't work properly.
High-speed communication between the PICs
I use SPI to communicate data in a half-duplex manner from one PIC to the other. Since the communication is unidirectional (from the PC to the display), I don't have to worry about collisions. I still have the other wire hooked up, just in case I decide to upgrade the functionality later.