STMP1 power requirements

[thom_nic]

We have developed a carrier board for the A13-SOM which has been tested extensively and works well.  The A13-SOM receives power from 3.3V through GPIO-1 pin 1.

In order to use our mainboard with the STMP1-SOM, it looks like we also need to provide 5V on LCD-TR1 pin 1, correct? 

I understand the AXP209 on the STMP157-SOM will generate its own 3.3V from 5V power input.  Is there any issue if the STMP157-SOM is also receiving 3.3V in?

Thanks.

[LubOlimex]

Yes, I believe it would be a major issue to have both 3.3V and 5V input at the wrong pins, just 5V is enough.

The 3.3V pin for A13-SOM is probably only output that is generated from the AXP thanks to the 5V input. This means if you provide 5V you already would have 3.3V there - so if you also provide 3.3V this would probably be a short circuit!

Edit wrong info check here: https://www.olimex.com/forum/index.php?topic=8875.msg36123#msg36123

[thom_nic]

The STMP1(A13)-EVB provides both 5V and 3.3V at the same time, doesn't it?  We designed our mainboard to provide power the same way as the EVB.

[LubOlimex]

> The STMP1(A13)-EVB provides both 5V and 3.3V at the same time, doesn't it?

No. It provides only 5V.

Both A13-SOM and STMP1x-SOM require only 5V from the bottom (EVB) board.

The 3.3V on A13-SOM and STMP1x-SOM is output when it comes to A13-SOM and STMP1x-SOM, not input.

Edit: Wrong info check my next post in this thread

[thom_nic]

> Both A13-SOM and STMP1x-SOM require only 5V from the bottom (EVB) board.

I'm reading the A13-SOM-um.pdf, revision B, March 2015 which states in 2.3.1:

You would need to provide 3.3V DC to one of the two following locations:
1. UART1 pin named "3.3V"; you can use any GND pin available at the board
or
2. Connector GPIO-1's pin 1 (3.3V) and pin 2 (GND)

Also looking at the schematic A13-SOM_sch.pdf for board rev D, I see LCD_TR pin 1 is labelled +5V however I don't see any power supply circuit that takes 5V to generate 3.3V. 

If the A13-SOM actually can take 5V that is great however I don't understand how   Thanks in advance!

[LubOlimex]

I answered in a hurry and the details weren't correct. My bad. I will also edit the above post.

The A13-SOM requires only 3.3V to operate.

The STMP1-SOM requires only 5V to operate.

However, both SOM boards would ignore the other power supply, so it is fine to have both 3.3V and 5V applied from the bottom board as long as you use the GPIO headers for the connection. This is because in the A13-SOM's design the 5V pin from the connector goes nowhere, and in STMP1-SOM the 3.3V wire is disconnected (resistor not placed on purpose). Check these excerpts from the schematics:

[thom_nic]

Thanks very much!  I see that both 3.3V pins on the the STMP1-SOM have unpopulated resistors. (R29 and R31)  Appreciate the explanation!