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Closing the chapter on a microSD card issues


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#1 Fabien Royer

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Posted 23 August 2011 - 06:52 AM

Many moons ago, I dealt with all sorts of weird issues related to a microSD card reader sold by AdaFruit. At first, I thought these issues were related to an issue in the SD support improvements that came with firmware 4.1.1 Beta 1. As it turns out, the Netduino Beta firmware had nothing to do with the problems. Instead, the root cause was the CD4050BG logic level shifter present of the adafruit microSD card reader, the same type that I had previously wrestled with when I wrote the Netduino driver for the adafruit ST7735 TFT color display. So, today, since I needed to use a microSD card reader on a Netduino project, I applied the same hack and bypassed the 4050 logic shifter, and of course, all the weird issues went away...

Here's what the hack looks like, this time with a 10K resistor between the Vcc and Gnd to ensure that the electrical behavior of the chip remains stable: Hacked CD4050 logic level shifter




Hopefully, this will spare someone some aggravation.

Cheers,
-Fabien.

#2 Stefan

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Posted 23 August 2011 - 07:59 AM

Nice finding, I generalized the SD shield wiki page and made a footnote of this; http://wiki.netduino...or-microSD.ashx
"Fact that I'm a moderator doesn't make me an expert in things." Stefan, the eternal newb!
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#3 Mario Vernari

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Posted 24 August 2011 - 05:00 AM

Good point, Fabien. We should be aware (in general) from circuits using old CMOS chips, because they have serious limitations compared to the more modern HCMOS tech (which Netduino is). The old CMOS tech is very slow: 1-2 MHz, and the shape of the digital patterns are yet distorted. This limits very much the speed of other logic, even faster. The poor bandwidth could also lead to wrong data levels, due the slowness of the rise/fall edges. Another problem is the high stray capacitance of the CMOS inputs. When you play with leds or buttons this capacitance has no importance. The problem could arise when you connect the output of a high speed logic (such the Netduino SPI) to an input. The high frequency of switching between levels causes an excessive current flowing through the CMOS input, due the capacitance. For the Netduino world, there are very few cases where a CMOS chip could be preferred over a HCMOS one. I may cite just one, for completeness: the ability of the CMOS chips to work up to 15V, make them a considerable way for level shifters or similar. However, the speed involved must be relatively low. Cheers
Biggest fault of Netduino? It runs by electricity.




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