This document contains some technical details that might be helpful in finding problems for the communication between controller and the two panel PCBs.

SPI and I2C Communications

In this section we document the SPI and I2C communications used by the G4 panels.

Overview

Display data is sent from the controller to panels via SPI. Each panel, after receiving data, will display the pattern specified via the data exactly once (one-shot display update). In order to continuously display a visual pattern - whether static or changing - the panels must receive display data at a fixed clock rate from the controller. The exact frequency of this display update clock is not crucial, however for a consistent display intensity the clock rate should be constant. Also, for the two gray scale modes supported by the panels - 16-level and 2-level - there is as maximum display update clock frequency. Operating above this frequency is not advised as the panels will likely receive garbled data. The maximum display update frequencies for the two gray scale modes are:

16-level gray scale 500Hz
2-level gray scale 1500Hz

The duration of the one-shot display update performed by a panel is encoded in a byte sent with the display data for each LED Matrix. This duration parameter can be used to adjust the effective duty-cycle of the display and can thus be used to control the overall brightness of the matrix. Note, using a larger (than default=0) display duration parameter may decrease the maximum operating frequency.

For a given panel a display update is initiated when it receives data from the controller. The data is first received (via SPI) by atmega328 micro-controller on the comm sub-panel. The SPI message contains the display information for each of the 4-matrices on the driver sub-panel. Next, the micro-controller on the comm sub-panel splits the message into four equal sized each parts - one for each LED matrix on the driver sub-panel - and forwards each part (via I2C) to the atmega328 micro-controller on the driver sub-panel corresponding to LED matrix associated with the data. For newer custom made driver boards, the same communication scheme is kept. The micro-controller on the driver sub-panels will then display the data exactly once.

I2C Messages

In this section the format of the I2C messages sent from the comm sub-panel to the driver sub-panel is described.

The length of the message depends on the number of gray scale levels in the display pattern:

  • 16-level gray scale, length = 33 bytes
  • 2-level gray scale, length = 9 bytes

The first byte of the message is special and encodes the number of gray scales levels in the subsequent data and the display stretch parameter which determines how long the one shot pattern is displayed. The contents of the first message are as follows:

  • i2cMessage[0], bit 0 = gray scale identifier (0 = 2-level gray scale, 1 = 16-level gray scale)
  • i2cMessage[0], bits 1-7 = display ‘stretch’ parameter (0-127). 0 is the shortest possible display and 127 the longest.
i2cMessage[0] = GRAY_SCALE_ID | (STRETCH_PARAMETER << 1);

The remaining bytes in the message - 32 for 16-level gray scale or 8 for 2-level gray scale - consist of the display pattern data.

I2C: 16-level Gray Scale

For 16-level gray scale each pixel is encoded by four bits so that each byte of the message consists of the data for two pixels. Columns are encoded first as follows:

i2cMessage[1] = pixel[0][0] | (pixel[0][1] << 4);
i2cMessage[2] = pixel[0][2] | (pixel[0][3] << 4);
i2cMessage[3] = pixel[0][4] | (pixel[0][5] << 4);
i2cMessage[4] = pixel[0][6] | (pixel[0][7] << 4);
i2cMessage[6] = pixel[1][0] | (pixel[1][0] << 4);
i2cMessage[7] = pixel[1][2] | (pixel[1][3] << 4);
... 
i2cMessage[32] = pixel[7][6] | (pixel[7][7] << 4);

I2C: 2-level Gray Scale

For 2-level gray scale each pixels is encoded by one bit so that each byte of the message consists of the data for 8 pixels. Columns are encoded first as follows:

i2cMessage[1] = pixel[0][0] | (pixel[0][1] << 1) | (pixel[0][2] << 2) | (pixel[0][3] << 3) | ... | (pixel[0][7] << 7);
i2cMessage[2] = pixel[1][0] | (pixel[1][1] << 1) | (pixel[1][2] << 2) | (pixel[1][3] << 3) | ... | (pixel[1][7] << 7);
…
i2cMessage[8] = pixel[7][0] | (pixel[7][1] << 1) | (pixel[7][2] << 2) | (pixel[7][3] << 3) | ... | (pixel[7][7] << 7);

SPI Messages

In this section the format of the SPI messages sent from the controller to the comm sub-panels is described.

The SPI message simply consists of four I2C messages - one for each matrix on the panels - stacked one after the other. The data for matrix 0 is given first, followed by the data for matrix 1, etc.

SPI: 16-level Gray Scale

The bytes in the SPI message are as follows:

  • spiMessage[0-32] = i2c message for matrix 0
  • spiMessage[33-65] = i2c message for matrix 1
  • spiMessage[66-98] = i2c message for matrix 2
  • spiMessage[99-131] = i2c message for matrix 3

The total message length is 4*33 = 132 bytes.

SPI: 2-level Gray Scale

The bytes in the SPI message is as follows:

  • spiMessge[0-8] = i2c message for matrix 0
  • spiMessage[9-17] = i2c message for matrix 1
  • spiMessage[18-26] = i2c message for matrix 2
  • spiMessage[27-35] = i2c message for matrix 3

The total message length is 4*9 = 36 bytes.

SPI: Parameters

Parameter Value
Bit Order MSBFIRST
Data Mode Mode 0
Maximum Clock 4Mhz