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Topic: reading audio files from SD card waveshield (Read 3140 times) previous topic - next topic

Boffin1

I dont know if this is the right place to ask this but I have been using the waveshield based circuit on some devices to play sound clips from a SD card.

One unit failed, and while faultfinding I see that from the circuit diagram that I sort of copied from http://www.ladyada.net/images/wavshield/v11/wave11schem.png  that the 74ls125 is used as a buffer between the 5v data lines from the micro, to the 3v3 SD card.

What I didn't notice was that the 74125 is powered from 3v3 ( the data sheet says min Vcc as 4v75 ? )

The inputs to the chip are 5v but the outputs are less than 2 volts.

I don't know if this is part of the trouble, I havn't got it working yet, but has anyone had problems with this ?

Boffin1

I see that the one on the circuit states 74125N  which I cannot even get the specs of on the net ??

Grumpy_Mike


Boffin1

I don't know Mike , they mention the N suffix in many datasheet titles ( like the one you linked ) but do not actually mention it in the sheet, and everything in the sheet states the 4.7v minimum supply.

I recall seeing a post from the guy who wrote the wavehc library, about using the buffer instead of a resistor voltage divider ( as used in the adafruit first waveshield version )

I don't seem to be able to find that post now, but I might try the resistors tomorrow anyway..

Grumpy_Mike

Quote
and everything in the sheet states the 4.7v minimum supply.

Yes it will be

If you want a lower working voltage try the 74HC125N

Don't worry about the N suffix.

Boffin1

It will work fine with a 3v3 supply, but the input must be 0 - Vcc and the inputs are 5v from the micro....  perhaps I am missing something here.

http://www.cl.cam.ac.uk/teaching/2003/DigElec/part2-data.pdf


Grumpy_Mike

The 74125N has 5v tolerant inputs, that is it will work with 5V input when only being powered off 3v3.
After some grubbing around I think that part is no obsolete and now you should use something like this:-
http://uk.farnell.com/fairchild-semiconductor/74lcx244msa/ic-74lcx-logic/dp/1504625 

Boffin1

Thats the one it needs, thanks,  now to see if I can get it in Africa !

Boffin1

nearly two years lead time from Farnell  !  ,  I think I will play with the resistors, or perhaps a quad op amp.
But thanks for your time

Grumpy_Mike

OK try 74LCX125 they have those in stock and they have 5 v tolerant inputs.

Boffin1

Thanks Mike,  our local agent who mirrors Farnell only has the TSSOP package, but I might be able to get the SOP package through RS Components who operate here. ( I can solder that one, and the production run is too small to warrant outside assembly )

By the way, I got back to fault finding my faulty unit, and it turned out to be a faulty SD card ( 4 weeks old )

I hope addressing them with only 2 volts data doesn't kill them !   The other 3 units are working fine.

I might change the micro in the audio side of the unit to run from 3v3 to resolve the problem, and use a simple resistor/clamp circuit for the address lines which are very slow.
Thanks again for your time..

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