[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] 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).