225 Serial Character LED Matrix :: Quicker, easier and cheaper to make your own chip!

How it works

This project drives an LED-matrix character display composed of one or more Pixie Chroma chainable devices, each one featuring two 5x7 LED matrices based on the WS2812B RGB LEDs. Each 5x7 LED matrix displays one character. The display shows characters received over a serial port.

Up to 4 chained devices are supported for a maximum of 8 characters. The displayed characters are received from a serial port using the UART protocol (9600 baud, 8N1). The project includes a simple UART implementation. Every time a character is received over UART, the displayed characters shift left, and the new character appears to the right. 5x7 matrix representations for printable ASCII characters are supported using the font from Arduino Microview Library encoded in a character ROM. Non-printable ASCII characters are shown as an empty rectangle. Each new character appears with a color randomly chosen among a palette of 16 colors contained in a color ROM. A pseudo-random number generator based on a linear-feedback shift register is used for color selection.

The project is designed to demonstrate components that should be easily re-usable:

WS2812B LED strip driver
UART receiver and transmitter (8N1 only, no flow control)
linear-feedback pseudo-random number generator
character ROM
cocotb tests for UART and WS2812B protocol

How to test

Basic setup:

Connect uo[0] to the input pin (DATA_IN) of a Pixie Chroma LED-matrix display (two 5x7 WS2812B LED matrices). Ensure the VCC and GND pins are connected to an adequate power source.
Configure the input (e.g., using the DIP switches of the PCB) as follows: set ui[0] and ui[1] to 0 to use one Pixie Chrome (i.e., two 5x7 LED matrices); set ui[2] to 1 to enable UART echo; set ui[4] and ui[5] to 0 to disable LED dimming, set ui[6] to 0 to select internal display refresh, and ui[7] to 0 to select random color selection.
Connect the UART interface of the project (RX is ui[3], TX is uo[4]) to a serial terminal or a UART-to-USB PMOD or adapter (e.g., the one provided by the onboard RP2040 of the Tiny Tapeout PCB). Configure the serial interface for 9600 baud, 8 bits, 1 start bit, no parity bit, and 1 stop bit (8N1), with no hardware or software flow control.
Open the terminal and type any characters: printable ASCII characters will appear from the right-hand side on the LED matrix and shift left as more characters are typed. Each character will appear with a different random color. Non-printable ASCII characters are shown as an empty rectangle. When ui[2] is set to 1, received characters are echoed on the serial connection.

To use more than one Pixie Chroma, chain additional displays after the first one. This project supports up to 4 displays (e.g., 8 5x7 LED matrices). Set ui[1] and ui[0] (e.g., by using the DIP switches of the PCB) to configure the number of displays you are using: 00 for 1 display, 01 for 2 displays, 10 for 3 displays, and 11 for 4 displays (8 5x7 characters).

Dimming of the LED matrix is controlled by the ui[5] and ui[4] signals: 00 for no dimming, 01, 10, and 11 for increasing dimming.

The color of characters is randomly chosen when ui[6] is low, and fixed when ``ui[6]` is high. The fixed color can be changed by sending over UART a non-printable byte >= 128: the lower 4 bits of such value are stored and used as an index in the color ROM (16 different colors).

Two display refresh modes are available, controlled by ui[6]. When ui[6] is low, refresh is internally triggered at a frequency of about 75 Hz. When ui[6] is high, refresh is triggered via UART, whenever a CR or LF character is received. For both refresh modes, uo[1] is pulsed high on LED matrix refresh.

External hardware

1 to 4 Pixie Chroma WS2812B LED-matrix displays (chained if more than one is used). An external power source for the LED matrix is recommended.
UART terminal or UART-to-USB adapter (PMOD or on-board via RP2040). TX-only is sufficient.

#	Input	Output
0	num chars selector 0	LED strip
1	num chars selector 1	LED strip latch
2	UART loopback option
3	UART RX
4	dimmer selector 0	UART TX
5	dimmer selector 1
6	internal/external refresh selector
7	random/fixed color selector	UART RX valid

225 Serial Character LED Matrix