[LASMO] Started PCB design

This week, I start to draw the design of the PCB.
I start to add the main components of the PCB, and decide the exact references we need :

  • For the STM32F7, all references available on the ST website are suitable, so I choose the one that was already in the Xpedition PCB library, the STM32F767vit6
  • For the ESP32, Luc choose the ESP32-WROOM-32: it has an embedded Wi-Fi antenna, an RMII interface for the ethernet, and an SPI interface to communicate with the STM32F7
  • For the security microcontroller, I choose the STM32F103RE. This MCU has to integrate 5 ADCs, one GPIO and an SPI interface. So, a very small STM32 MCU is enough.
  • For the triple 8bits-DACs, I change the MAX512 to a MAX5105 because the MAX512 don’t integrate three equal DACs. Moreover, the MAX5105 has an asynchronous MUTE input, which is convenient for the stop of the LASERs by the security MCU

Then, I work on the Ethernet connection. I add a LAN8742A and an RJ45 connector to make the physical layer. I never made design PCB, so I spent a lot of time to find and understand the different components to add to the design.

After finishing the Ethernet design, I work on the MAX5105 connections. This part was easy to make, but I’m not sure about the direct connection (cf figure 1) between the DACs’ outputs and the inputs of LASER drivers. I don’t have the LASER documentation yet, so it’s confusing for me to know how correctly connect these two parts.

(figure 1) MAX5105 connections

This is the newest version of our main components implementation :

[LASMO] Main components architecture

The LASER and the scanner were chosen : a LASER RGB of 1W and a scanner with a speed of 30 Kpps. They came from laboutiquelaser.fr, a French website. We are waiting for the technician’s answer for the availability of the LASER and a real documentation of each component. 

The choice of these two components allow us to define the main architecture of the project :

Main components’ architecture

The main controller is a STM32F7 family. On this micro-controller we will :

  • Use two DAC of 12 bits to control the scanner
  • Use two ADC of 12 bits to get a stereo input line ( in order to synchronize the animation with a sound beat )
  • Communicate with a SD card 
  • Communicate with ESP32, STM32F3 and MAX512 with SPI protocol

The network controller is an ESP32. It will allow us to communicate with LASMO over Ethernet or Wi-Fi. It will integrate a HTTP server to provide us a web app in order to control LASMO.

The MAX512 is a triple 8 bits DAC. It will be able to control the LASER with 3 analog inputs (RGB). This information have to be confirmed, we don’t have yet the documentation of the LASER. In case the input is not analog, the MAX512 component will be removed.

The STM32F3 is a micro-controller whose solely role is to comply with security norms : indeed, the LASER can’t be turn on more than 25 ms on the same position. The security controller monitors the return position signal of the galvanometers and force the laser inputs signals to zero if the galvanometers don’t move enough.

[LASMO] DAC constraints

The galvanometers are controlled by an analog signal input (between -5V and +5V). So, we have to use DACs in order to convert each digital coordinate. In the ILDA Format, each coordinate are code on 16 bits, but very few microcontroller embed 16 bits DAC. It’s generally DAC of 12 bits we can find on common microcontroller (like STM32 series).
With 12 bits on each coordinate, the display resolution is 4096×4096, witch is bigger than the 4K resolution (3840×2160) ! 12 bits for each DAC are quite enough.

Most of DAC on STM32 can operate up to 1 Msps (megasamples per second) and it’s compatible with our 30Kpps (and so 30Ksps).
Most of DAC on STM32 have an output signal voltage of approximately VDD=~3,3V. If we want an -5V/+5V range, we must use an AOP. For this king of gain (<10), most of AOP can easily operate at our speed ( 30Kpps ).

In conclusion, it will be easy to find a microprocessor with DAC  requirements. The only constraint is the number of DACs: a minimum of two for the two axes but a third is required if we want use an analog LASER control (in opposite of TTL control).


Informations on this post are taken from ILDA official website.
The ILDA format is intended for frame exchange purposes only. It is not optimized for space or speed, and it is not currently concerned with display issues such as point output rate. Also, the format does not include show information such as timing of frames. Generally, the highest function the ILDA format can provide is a sequence of frames which play back to form an animation.
The ILDA File can provide a 3D or 2D structure, and provide color with Indexed color in a table or True colors on 24 bytes. In order to best estimate the required useful bit-rate, we analyse the most disadvantageous case : format 4 with 3D points and “True Color”. All these numbers are for one frame and N points per frame :

  • a header of 32 bytes
  • N data records of 10 bytes :
    • 2 bytes for X coordinate
    • 2 bytes for Y coordinate
    • 2 bytes for Z coordinate
    • 1 bytes for status code
    • 3 bytes for “True Color” ( 1 bytes red, 1 bytes green, 1 bytes blue )

With frame_rate in fps, N in points per frame and k in pps :

  • N = k / frame_rate
  • bit_rate_per_frame = 32 + 10*N
  • bit_rate = bit_rate_per_frame * frame_rate

For our project, we can assume that the frame rate will not exceed 60fps (and generally 25fps) and the number of points will not exceed 30Kpps (the commercial galvanometers can’t go faster). So, the bit-rate of an ILDA animation will not exceed 300KB/s.

[LASMO] Start specification of our project

In order to choose the components of our project, I decided to focus on the format of a LASER animation: ILDA. Indeed, the format of the animation requires us the bit-rate of our data flow, the time-precision and spatial accuracy of the galvanometers.

These informations are essential for the choice of all the components: the class of the SD card, the generation of the USB protocol, the choice of the Wi-Fi and ethernet specification, the requirements of DACs (to control galvanometer and LASER), and finally the choice of the micro-controller. 

In order to collect all these informations, I created the Wiki of our project LASMO. I put the information about the ILDA Format, and the information of one of a galvanometer we will be able to use.