Constant voltage from BNC/DAC ports

[João Pedro Junqueira]

Hello everyone,

I've been having an issue with 2 BNC ports and 1 DAC. I can't get them to work as intended as outputs or inputs. When I measured their voltage, I got: BNC 1 - 2.30V; BNC 2 - 0.43V; DAC 2 - 1.67V (even when the system was disconnected from my PC). I figure we might have damaged then when we connected a pulse output from a laser pulse generator which sends user-selected voltage. If that's the case, is repair of these ports possible or relatively straightforward?

Thank you,

Joao.

[thoma...@neuro.fchampalimaud.org]

Hi Joao,

Only the DAC 1 and DAC 2 BNC connectors can be used for analog output. To use them as analog outputs you need to instantiate a pyb.DAC object.

DAC1 = pyb.DAC(1) # Instantiate DAC object.
DAC1.write(0)          # set the voltate to 0.
DAC.write(255)        # Set the voltate to its maximum value (by default DAC is 8 bit but can be specified to be 12 bit, see docs linked above).

Did you do this before doing the voltage measurements?

All the BNC connectors on the breakout board connect directly to pins on the pyboard microcontroller, so it is in principle possible to damage them by applying too much voltage. I don’t think the voltage measurement you did so far are definative though as unless you have specified them as an output and set the voltage they will be floating. I suggest testing them using the standard pyControl digital input/ouput classes to see if they are OK. If they have got damaged you can replace the pyboard, as it is connected to the breakout board using connectors rather than being soldered on.

best,

Thomas

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[João Pedro Junqueira]

Hello,

Thank you for the reply. I actually only needed them for digital input/outputs. I'll test them with the standard pyControl classes.

But if I'm to replace the pyboard, is the 1.1 version the only one compatible with the breakout?

Best,

Joao.

[thoma...@neuro.fchampalimaud.org]

Hi Joao,

I assume your asking because the pyboard v1.1 is currently out of stock from the micropython store.  The board that they have in stock, the pyboard lite v1.0 is probably not a good option because it has less ram and flash storage space than the pyboard v1.1 (see comparison here), so you might well have issues running out of space or memory.  There was a board called the pyboard v1.0 (not lite) which had identical specs to the pyboard v1.1 but a couple of different components (e.g. the voltage regulator) and that one would work fine, but does not appear to be sold any more.  

I have contacted micropython to ask them when they expect the pyboard v1.1 to be back in stock and will post their reply here.

There are some boards that claim to be direct replacements for the pyboard v1.1 available from Chinese suppliers, e.g. this.  I have not tried these but they might be an option until official stock is available.

If it seems that there is going to be a long duration supply disruption for the pyboard v1.1 I will design an adapter to allow the pyboard D series to be used with micropython breakout boards, as these are currently in stock and more powerful than the pyboard 1.1.

best,

Thomas

[thoma...@neuro.fchampalimaud.org]

I heard back from Micropython regarding availability of the pyboard v1.1.  They said that they can't currently source the microcontroller chip due to supply chain disruptions, but will resume production of the pyboard v1.1 as soon as supply of the chip is available again.  So no firm date but definitely future production once the supply chain mess resolves.  See also this video.. https://blog.adafruit.com/2022/05/05/chip-shortage-st-stm32f405-chipshortage-adafruit-adafruit-st_world-micropython/

[João Pedro Junqueira]

Thank you for the info. We ended up ordering another breakout board, which might be useful for parallel experiments anyway. But I'll keep on eye on their

site.

Thank you,

Joao.

[João Pedro Junqueira]

Hello again,

Regarding the possibility to replace the pyBoard on the breakout board: how would I carry that out? Is it possible to remove it without desoldering it from the  header/receptacles?

I'm not too experienced on this.

Regards,

Joao.

[Thomas Akam]

Hi Joao,

Apologies for the delay getting back to you, I missed your message while I was on holiday.  The pyboard is connected to the breakout board using connectors, not soldered on directly, so you don't need to do any soldering to remove it.  You just need to pull the pyboard away from the breakout board.  It is possible to accidently bend the connector pins on the on the breakout board when you do this, so to avoid this I normally lever it off gently by putting a pen or screwdriver between the pyboard and the breakout board. 

You will need to solder female header connectors onto the new pyboard.  The connectors we use are listed in the breakout board BOM and are Samtec CES-106-01-T-D, CES-116-01-T-S, but any female header connector with the same pitch and pin layout should work fine.

best,

Thomas

[João Pedro Junqueira]

Hello Thomas,

Understood! Thank you for the support. I'll try the replacement.

Best,

Joao.