LichtKrant

Project LichtKrant
Controller for 3-colour 80x7 pixel moving LED sign
Status	In progress
Contact	bertrik
Last Update	2019-12-23

Introduction

Found a 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 ESP, 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?

Short term improvements:

• pixelflut server
• generic receiver process for image frames (80x7 RGB, in UDP or TCP/IP)

Reverse engineering

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.

How does it work

Finding out how it works is done by tracing out the connections from connector J1, which connects the processor board to the LED board.

*  J1.1: IC2.A1 -> IC2.B1 -> via ... -> shift clock
                -> IC1.B1 -> IC1.A1 -> U11.SRCLK
                -> IC1.B6 -> IC1.A6 -> U1.SRCLK
*  J1.2: GND
*  J1.3: IC2.A2 -> IC2.B2 -> via ... -> latch clock
*                      IC1.B2 -> IC1.A2 -> U11.RCLK
*                      IC1.B5 -> IC1.A5 -> U1.RCLK
*  J1.4: GND
*  J1.5: IC1.B3 -> IC1.A3 -> U11.SER -> data pin for upper shift register (green)
*  J1.6: GND
*  J1.7: IC1.B4 -> IC1.A4 -> U1.SER -> data pin for lower shift register (red)
*  J1.8: GND
*  J1.9: IC2.A7 -> IC2.B7 -> via ... -> row enable
         IC4.1A -> IC4.1Y -> IC3.G2B
* J1.10: VCC
* J1.11: IC2.A3 (row) -> IC3.A -> row select
* J1.12: VCC
* J1.13: IC2.A4 (row) -> IC3.B -> row select
* J1.14: VCC
* J1.15: IC2.A5 (row) -> IC3.C -> row select
* J1.16: J4 (IR receiver)
* J1.17: IC2.A6 -> IC2.B6 -> IC3.G2A -> row enable?
* J1.18: ???
* J1.19: ???
* J1.20: connected to the "ethernet" dipswitch

• 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
• U1-U10 = lower chain of 74HC595 (8-bit shift register)
• U11-U20 = upper chain of 74HC595 (8-bit shift register)

The 74HC138 has an enable consisting of three signals G1, G2A, G2B. Signal G1 is connected to Vcc. Signal G2A is connected to B6 Signal G2B is connected through a hex inverter (IC4.1Y) -> IC4.A -> IC2.A7 -> IC2.B7

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

Timing

Some measurements on the timing of the signal with the original control board connected.

• The bit shift clock is about 300 ns high, 300 ns low. Frequency about 1.67 MHz.
• Row bytes are sent with an 9.6 us interval.
• The column latch clock pulses with an interval of 384 us, it's a positive going pulse about 300 ns long. This means a line rate of about 2.6 kHz.
• Row enable on J1.9 has an interval of 384 us, with a low-time of about 80 us.
• Row enable on J1.17 has an interval of about 6.16 ms, with a low-time of about 3.08 ms. This is relatively slow. My guess is that this is a kind of global brightness control PWM signal.

Columns:

• J1.11 (row select): 0.37 ms high, 0.37 ms low
• J1.13 (row select): 0.76 ms high, 0.76 ms low
• J1.15 (row select): 1.5 ms high, 1.5 ms low

-> the rows cycle through 8 states (although there are only 7 rows) -> the entire cycle takes about 3 ms, so the frame rate is about 300 Hz -> each row is enabled for 0.37 ms, consistent with the row column latch interval

Power consumption

The duty cycle of the LEDs is strongly determined by the row enable pins. It appears that J1.17 is used by the original control board to set the global brightness.

• all LEDs off, about 20 mA
• all red LEDs on, about 4.0 A
• all green LEDs on, about 3.7 A
• all LEDs (red + green) on, about 6.4 A

Software

Preliminary code can be found on the github lichtkrant repository.

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).

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.

Alternative: Run the real-time part in an interrupt, leave the "main-loop" for non time-critical tasks. This is what is implemented in the esp8266 branch of the software archive, currently targeted at an ESP8266 even.

variable pixel brightness

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, for each pixel, we add the brightness value from the frame buffer to the brightness count (e.g. 0-255). If the addition results in an overflow, the pixel is lit on the display, otherwise it is dimmed.

 For example, adding a brightness value of 128 causes the addition to overflow every other time, causing a 50%/50% duty cycle at half the update frequency.

• 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).

frame rate

The achievable frame rate is limited by the shift clock rate, suppose this is 1 MHz. One frame takes 80x7 = 560 clocks, or 0.56 milliseconds, or equivalently the framerate is 1785 Hz maximum.

• a pixel at 1/256 brightness (lowest) would flash with a rate of about 7 Hz.
• a pixel at 4/256 brightness would flash with a tolerable rate of about 28 Hz.
• a pixel at 8/256 brightness would flash with a tolerable rate of about 56 Hz.
• a pixel at quarter brightness (64/256) would flash with a rate of about 446 Hz.
• a pixel at medium brightness (128/256) would flash with a rate of about 892 Hz.

So it looks like we can get about 5 bits of brightness (32 levels) out of it and still have tolerable flashing rates (higher > 50 Hz).

Links

Possibly useful other info:

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