This was an educational project I undertook to learn more about PCB design and programming microcontrollers. The design goal was to produce a low cost alphanumeric display easily readable from 50ft outdoors in bright sunlight.
TL;DR:
- Schematic
- Layout
- Firmware, Startup Code, and Linker Script
- Driver
- Gerber, BOM, and CPL
The LEDs are wired in parallel, with P-channel MOSFETs on the rows and N-channel MOSFETs on the columns. The screen runs at a refresh rate of 1.25KHz, or a row scan of 7.5KHz, with the goal of avoiding rolling shutter artifacts in photos and videos. An ARM Cortex M0 drives shift registers to control the columns using its SPI peripheral, and switches the rows with dedicated GPIOs.
The board is programmed via a TTL UART, with a 48 byte LSB-first row major representation of the pixels on the board, left to right.
The driver.py script is an example of how to drive the display. The font in ledfont.py is the 4x6 terminal font from the Linux kernel.
Datasheets:
- ARM M0 STM32F030C8T6 (see also RM0360)
- 12MHz Cyrstal X322512MOB4SI
- 3V LDO SK1117-3.3
- Switcher MT2492
- Shift Register 74HC595D-118
- P-Channel MOSFET AP2301B
- N-Channel MOSFET 2N7002AK
- LED HL-A-3014H416W-S1-08HL-HR3
Revision 2.0 realized a significant reduction in cost by replacing the expensive open drain shift registers with N-channel MOSFETs small enough to be driven by logic level registers. That also enabled removing the 5V level shifter and associated 5V LDO.
The switcher inductor value was far too large, and was limiting the brightness of the display: since ripple current isn't really relevant, I replaced it with the smallest inductor the switcher IC datasheet allows (22uH -> 4.7uH).
The PCB footprint was reduced from 365x50mm to 350x50mm.
I had 5 revision 2.0 boards manufactured and assembled by JLCPCB in December 2022: the cost was $24.50/each, plus $30 in shipping costs.
Revision 2.1 will use cheaper more widely available LEDs.
Adjustable voltage wall adapter: https://www.amazon.com/dp/B01ISM267G
No external libraries are necessary: everything is included here, and can be flashed using standard open source tools.
To build on Debian/Ubuntu, run:
sudo apt install make gcc-arm-none-eabi openocd
make
To flash to the hardware, attach the ST-LINK and run:
./flash.sh
Attach GDB with:
gdb-multiarch ./ledboard.elf -ex 'target extended-remote localhost:3333'
ST-LINK: https://www.amazon.com/gp/product/B01J7N3RE6
The license for the header files from STMicro can be found at include/LICENSE. Everything else is in the public domain, see LICENSE in this directory.
Unless expressly stated otherwise, the person who identified the work makes no warranties about the work, and disclaims liability for all uses of the work, to the fullest extent permitted by applicable law.