LichtKrant

Project LichtKrant
350px
Some 3-colour 80x7 pixel moving LED sign
Status	In progress
Contact	bertrik
Last Update	2019-12-17

Introduction

Found this moving LED sign on the revspace free stuff table.

It is 80 pixels wide, 7 pixels high. Each pixel has two sub-pixels (red and green), so it can display 3 colours (off, red, orange, green)

Goals:

• reverse engineer the electronics and document how it works
• replace the controller board with an ESP32, so we can receive image/text updates over the network
• implement more colour depth, so we can display gradient bitmaps (not just red, green, orange)
• write software for this, so other people can do the same for their display

Ideas:

• replicate the existing 80x8 display at revspace, but with one less row and no blue LEDs
• implement a pixelflut server, e.g. pretend to be RGB and just skip the blue channel
• double buffering, or some other kind of sync mechanism/callback?

Hardware

lichtkrantconnector

It consists of a processor board and a LED board.

Processor board

The processor board has a 8052, a flash chip, battery, 32768 Hz crystal, 40 MHz crystal, Altera Max chip. It appears the Altera chip drives two 245 chips which in turn drive the signal to the connector J1 to the LED board.

It takes 5V as input.

The PCB has a text on it: HXITC017_2.PCB

LED board

The LED board contains the actual pixels, organized in 16 modules of 5x7 LEDs. Each LED has a red sub-pixel and a green sub-pixel, so it can do colors: red, green, orange. The LED modules have the text GYXM-G2357ASRG on them.

The PCB has a text on it: HXBI80070509.PCN, date 2007-12-28

The are already electronics on it, consisting mostly of 595 chips (serial shift registers). The are two chains of 10 pieces of 74hc595 chips each, controlling the columns. There is an '138 chip (3-to-8 decoder) that drives a set of MOSFETS, 7 in total, controlling the rows. The MOSFETs are SDM4953, dual P-channel MOSFETs.

Reverse engineering the LED board electronics

This is done by tracing out the connections from connector J1, which connects the processor board to the LED board.

*  J1.1: pin IC5.A8 -> IC2.A1 -> IC2.B1 -> via ... -> shift clock
                    -> IC1.B1 -> IC1.A1 -> U11.SRCLK
                    -> IC1.B6 -> IC1.A6 -> U1.SRCLK
*  J1.2: GND?
*  J1.3: pin IC6.A1 -> IC2.A2 -> IC2.B2 -> via ... -> latch clock
*                      IC1.B2 -> IC1.A2 -> U11.RCLK
*                      IC1.B5 -> IC1.A5 -> U1.RCLK
*  J1.4: GND?
*  J1.5: pin IC6.A2 -> IC1.B3 -> IC1.A3 -> U11.SER -> data pin for upper shift register
*  J1.6: GND?
*  J1.7: pin IC6.A3 -> IC1.B4 -> IC1.A4 -> U1.SER -> data pin for lower shift register
*  J1.8: GND?
*  J1.9: pin IC6.A4 -> IC2.A7 -> IC2.B7 -> via ... -> row enable
                    -> IC4.1A -> IC4.1Y -> IC3.G2B
* J1.10: VCC?
* J1.11: pin IC6.A5 -> IC2.A3 (row) -> IC3.A -> row select
* J1.12: VCC?
* J1.13: pin IC6.A6 -> IC2.A4 (row) -> IC3.B -> row select
* J1.14: VCC?
* J1.15: pin IC6.A7 -> IC2.A5 (row) -> IC3.C -> row select
* J1.16: ???
* J1.17: pin IC6.A8 -> IC2.A6 -> IC2.B6 -> IC3.G2A -> row enable?
* J1.18: ???
* J1.19: ???
* J1.20: connected to the "ethernet" dipswitch

IC2.A7 <- IC4.1A <- via
          IC4.1A -> IC4.1Y -> IC3.G2B

• IC1 = 74HC245 (octal buffer) on LED board
• IC2 = 74HC245 (octal buffer) on LED board
• IC3 = 74HC138 (3-8 mux) on LED board
• IC4 = 74HC04 (hex inverter) on LED board
• IC5 = 74HC245 (octal buffer) on CPU board
• IC6 = 74HC245 (octal buffer) on CPU board
• U1-U10 = lower chain of 74HC595 (8-bit shift register)
• U11-U20 = upper chain of 74HC595 (8-bit shift register)

There are 8 rows (3 bits) controlled by the 74HC138 and 8 MOSFETS and two chains of 10x 74HC595 (8-bit shift registers) for the columns.

In summary, connector J1 has the following signals:

• columns
  • 1 bit shift clock shared for both shift registers (J1.1)
  • 1 bit latch clock shared for both shift registers (J1.3)
  • 1 data bit for 80-bit shift register 1 (J1.5)
  • 1 data bit for 80-bit shift register 2 (J1.7)
• rows
  • 2 bits row enable (J1.9 and J1.17)
  • 3-bits row select (J1.11, J1.13, J1.15)
• other
  • 5V signals
  • GND signals
  • some signal for the infared remote control

Levels on the bus are 5V.

Theory of operation:

• software writes two rows (red and green) of 80 bits into the shift register, using the shift clock lines and two SER lines
• software latches the data from the shift registers into the LED row output registers
• software selects a row using the three rows select lines and the row enable signals
• wait a few ms while the LED row it lit
• next row

Software

The plan is to drive the display using an ESP32, which is a dual-core processor. One core is dedicated entirely to driving the display in real-time so the other core is free to do other non time-critical tasks (e.g. receiving messages over the network).

Preliminary code can be found on the github lichtkrant repository.

Variable pixel brightness is planned using a kind of duty-cycle/dithering modulation:

• For each pixel we keep a running brightness count (e.g. 8-bit).
• At each frame start, we add to it the brightness value from the frame buffer (e.g. 0-256). If the addition results in an overflow, the pixel is lit on the display, otherwise it is dimmed.
• The running brightness count is initialized with a random number, this avoids synchronized flashing of the pixels on the display. In other words, pixels with the same brightness have the same duty cycle, but have staggered turn-of/turn-off times with respect to each other.

This is a kind of real-time dithering algorithm, the brightness count represents the error between the displayed brightness (on or off) and the desired brightness (value 0..255).

Communication between the cores consists purely of the state of the frame buffer. The main core writes the frame buffer, the real-time display core reads the frame buffer and updates the LEDs.

Links

Possibly useful other info:

• https://medium.com/@tharindu_peiris/p10-led-display-panel-using-esp8266-wemos-d1-mini-nodemcu-1e463ad60722