[LASMO] Making progress in PCB, basic I/O functions

Hi! Last week has been mostly about PCB design. We’re still waiting for our galvanometers and lasers to arrive… Hopefully we’ll have them soon. In the meantime we can’t make much progress about the PCB itself but we corrected some problems.

About the input of the galvanometers, I made a mistake in the voltage adaptation circuit: when the scanner is at its maximum angle, it should be either at -5V/5V or 5V/-5V (positive input/negative input). So we had to adapt the system so that when the ADC outputs 0V, it outputs -5V/5V; when 3.1V (max) 5V/-5V; when 1.55V, 0/0. That means we have to change the resistors values: R1 = 1kΩ, R2 = 2.21kΩ. As Mr Polti pointed out, we also have an issue with the component used: the ADA4941-1 has an output voltage swing of +/-2.9V, which isn’t enough to generate the desired signal. So we decided to make our own adaptation circuit with two ampops that will have the good characteristics (for example, the ADA4666-2 works). The circuit is as follow:

Voltage amplification circuit for scanner input

Meanwhile, I also worked a bit on coding the basic I/O functions of the main controller: I started with connecting the SD slot, testing my code on the development boards we use in practical works this year, OLIMEX STM32-E407. I haven’t been able to test it though, as the only microSD card I have is the on in my smartphone, and I failed to get it out of the phone.  

[LASMO] SD card and galvanometers in the PCB design

Because finishing the design of the PCB is now the priority task, I’ve been working on several parts of it: namely, linking the SD card connector and the galvanometer drivers.

MicroSD connector

The microSD card connector we found in ExpeditionPCB is manufactured by JAE (Japan Aviation Electronics). The pins correspond to those of the microSD standard and are linked to one of the SPI interfaces available on the STM32F7. MicroSD cards can work on supply voltage from 2.7 to 3.3V, so we just use the same 3.3V supply as the F7.

Our SD card connector’s connections…
… and the pins it’s connected to on the F7 

Galvanometer drivers

The drivers of the galvanometer work with a balanced analog input ranging from 0 to 5V. Since the DAC of the F7 outputs a single-ended signal ranging from 0 to 3.1V (according to the datasheet), this is an issue we had to resolve.

We basically had to convert an analog single-ended signal to a balanced one, and amplify it with a 5/3.1 ratio. We achieved this by using an ADA4941-1 amplifier with two resistances, as represented below.

Signal adaptation circuit between the DAC’s single-ended output (GALVA_X signal) and the driver’s balanced input (GALVA_DRIVERX_IN – and +). The two resistors inside the ADA are of the same value.

This circuit is composed of a non-inverting amplifier (taking IN and FB as entry), which, with the resistances of 10kΩ and 6.2kΩ, multiplies the signal by 1+3.1/5 = 1.62 = 5/3.1 (approximately). Then a inverting amplifier with a gain of 1, create a signal symmetric to the OUTP signal with respect to the REF signal value. We set REF at 5V so that the signal can have a 5V range around the offset value of 5V. That’s also why we linked the REF signal to the GND port of the driver, so that the driver can see the signal as a balanced signal.

[LASMO] Storage devices

LASMO will need to get data from the storage devices where the animations will be stored. For that reason, we need to add to our PCB a USB connector and a SD connector, as well as identify the electrical needs of the devices once connected and active. So I looked for what kind of SD cards and USB drives were fitted for LASMO. (streaming of  ILDA files). Thanks to Pierre, we know that we’ll need a debit of approximately 500 kbps.

SD card

SD cards are divided in 3 categories according to their storage capacity, and are granted a speed class among 11 possibles. 

https://www.sdcard.org/developers/overview/speed_class/

According to this table, all the speed classes on the market are fitted for the application.

As for power supply, it is kind of standardized. Indeed, accessing SD cards with an SPI bus requires a voltage of 3.3V. After looking at the cards available for example on Mouser, the typical supply current is between 30 mA and 100 mA. 

USB drive

Thanks to the presentation on USB we had a moment ago, we know that USB 2.0 and USB 3.0 deliver theoritical debits up to 480 Mbps and 5Gbps respectively. That is largely enough for what we need.

We had arguments over which type of connector will be the best (between USB-A and USB-C) . We finally decided to go with the most common (for the moment) USB-A.

As for the power supply, if the voltage is almost always 5V, the current does change with the version of the protocol: maximum 500 mA for USB 2.0 and  maximum 900 mA for USB3.0.