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Topic: driving SD directly with Uno. (Read 857 times) previous topic - next topic

ardnut

Aug 08, 2012, 05:59 pm Last Edit: Aug 14, 2012, 03:25 pm by ardnut Reason: 1
Hi, I'm looking at driving SD card directly with Uno rather than buying a break out board since it seems an unnecessary addition for so little and because it will take a while to get one.

I have a hex Schmitt 40106 that I was planning to use for level shift. Running the 40106 off 3v3 and resistor dividing the three lines coming from the UNO.  

Is that likely to be reliable?

I see there is a tiny breakout board that also uses low dropout regulator to proivde 3v3 for the card. Does that imply that UNO's 3v3 does not have enough current to power an SD card?

In fact the that question should be where is spec for Uno, other than the Atmel datasheet for the chip?

thx.

pito

1. the UNO - 3v3 vreg is 50mA rated, the max current an SDcard takes is around ~40mA peak
schematics http://arduino.cc/en/Main/ArduinoBoardUno/
2. 40106 - shall work from 3-15V, inputs not 5V tolerant (clamping current up to 10mA) so you need dividers (bad). It is very slow at 3V3 however - 60-90ns prop delays typ. @5V, even more @3V3. You need 2 in series (it is an invertor), so not sure ho it would work actualy..
3. When using resistor dividers you do not need the 40106 (good)..
p.

Grumpy_Mike

Another way of level shifting is to use a 74LS07 from 5V, it is open collector so use a 1K pull up to 3V3 on the outputs.

pito

.. the output of the lever shifter when sourced from 3V3 is not critical, what might be an issue is the input of the level shifter. Most ttl/cmos chips have so called "clamping diodes" at the inputs, which, when the voltage at the input is higher (ie 5V) than the VCC voltage of the chip (ie 3V3) conduct the current from its input to VCC (input->diode->VCC of the chip). The voltage difference (5V-3V3) is high enough to open such clamping diode full..

CrossRoads

74HC4050 used pretty often for level shifting.
An example:
https://www.sparkfun.com/products/9802

Latest Uno has seperate 3.3V regulator for higher current supply.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

ardnut

Thanks for all the replies.

4050 is probably a better solution I just did not have one, I was looking to use what I had in the box rather than wait for an order.

I've found Teensy do an SD shield at a very sensible price so I'll order that as a first stop solution.

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