Since the last episode, a lot of drastic changes occur in the FPGA architecture.
SCLK, LAT and GCLK
Those three signals are now generated by the SYNC module. This reduces the number of GCLK, LAT and GCLK sent outside the FPGA. They will be duplicated thanks to clock buffers on the “highway” PCB.
We decided to realize this change to reduce the number of pins used, this will make the main PCB easier to route. Moreover, to ensure proper functioning, the 20 Led band controllers had to send the same SCLK and GCLK and to send all the data for the following LEDs when the previous ones are displayed.… Read more
We decided to use the TLC5957 LED driver of Texas Instruments with Broadcom RGB LEDs: ASMB-KTF0-0A306.
First, we have to determine the maximum current which will be used. The 3 colours of the RGB LEDs can get a 20 mA input current max and the driver can handle 20mA sink current max. So we choose a maximum current of 20mA.
We decided to configure the driver with the maximum value for the Brightness Control (BC) (and by consequence, the maximum gain) to provide the intensity to go over 20mA. So we have to choose a value of BC equals to 7.… Read more
LitSimulate is a simulator which simulates what LitSpin will display. We developed the first versions with PyOpenGL and PyGame.
Each 3D image is represented by a classical image. If we note r the radial resolution (total number of columns), h the number of LEDs by column and n the number of steps by rotation, our image dimensions are rh x n. In our case r=20, h=32 and n=128. So we have 640×128. One line represents the n points one led will display. And each block of h lines represents the 360° image of a column from the outermost to the innermost (from 1 to r).… Read more
We discovered our first model contained a lot of issues. The outermost PCB often hide PCB behind them as you can see below. This results in some voxels being invisible. So we decided to create a simulator on Python to find the invisible areas. We will use it to determine the optimal configuration for our display. This configuration has several parameters such as the number of PCB, their position and the arrangement of the LED on one or both sides.
This a an example with 8 double-sided PCB in double spiral and 100 angular resolution. The blue dots are the visible voxels.… Read more