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Author Topic: I need help - I need an idea for level shifting  (Read 2681 times)
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LabVIEWguru
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« on: May 26, 2010, 07:37:08 07:37 »

Gents:

I'm working on a home project (since I don't have a job) using a Freescale 68HC908QC16, Atmel SPI memory and some other stuff at 3.3 volts. All is working well so far. I want to use a 7 segment, 6 digit display driven my a Maxim MAX7221 driver. This way I can send commands to the digit driver via SPI. The problem is that the max7221 spi is at 5 volt logic level, and the 68HC908QC16 SPI is at 3.3 volts. Of course the conversion has to be a "nice" square wave. I can't find any component made to do the level conversion. Transistor drive chews off the corners of the spi. I can't be the first person to have this problem, but I can't find any solutions - anyone have any ideas? Thanks in advance.

LabVIEWguru

Well, I just discovered the 74LC245 Level Shifter IC - guess I'll have to find a few. Any ideas out of junk-box parts?
« Last Edit: May 26, 2010, 07:50:08 07:50 by LabVIEWguru » Logged
sputnik
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« Reply #1 on: May 26, 2010, 07:50:45 07:50 »

The MAX6951 is the 3v counterpart to the 7221.  May be cheaper to just get the right part.
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hate
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« Reply #2 on: May 26, 2010, 12:08:09 12:08 »

A level shifter would be the best solution in my opinion but if u need a junk-box solution u can use an opamp in comparator mode to shift the voltage levels. That will be near perfect square wave. U will need to use an opamp with output voltage swing levels near to supply voltage like LM358 or so. Connect a voltage divider consisting of 2 resistors with an output level of about 1.5V to the inverting input, connect ur low voltage output to the non-inverting input and the output of the opamp will be ur input for the high voltage chip. That should work.

Regards...
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falcon _bd
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« Reply #3 on: May 26, 2010, 05:20:04 17:20 »

you can use this ckt


Or use this chip TXS0104E
« Last Edit: May 26, 2010, 05:22:09 17:22 by falcon _bd » Logged

titi
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« Reply #4 on: May 26, 2010, 05:58:23 17:58 »

Hi,

I think you need nothing to interface MAX7221.
If the MAX7221 works in 5v and its logic is in CMOS, so its low level is near 0v, so no problem for low level.
For high level, CMOS need upper VCC/2, so need above 2.5v, if SPI give 3.3v, you are above 2.5v that give good high level.
The simpless way is to check, there is no risk, and there is a good chance that it works.

Best Regards.

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sohel
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« Reply #5 on: May 27, 2010, 08:44:48 08:44 »

Here for PIC microcontroller. ODCx Register

http://pic24micro.com/general_purpose_input_output_introduction.html
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DreamCat
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« Reply #6 on: May 27, 2010, 04:33:51 16:33 »

if you only need a few port to do level shifiting,  you can use mosfet  done it.
here is a article:http://delphys.net/d.holmes/hardware/levelshift.html

for more port, such as bus, maybe SN74LVC4245, HCF40109 or other chip can help you.

you also can search this article:
AN97055_MOSFET Bi-directional level shifter Application Note
release by philps
« Last Edit: May 28, 2010, 06:37:54 06:37 by DreamCat » Logged

May be I expressed the wrong meaning, sorry for my bad english. Please correct it for me if you can.
LabVIEWguru
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« Reply #7 on: May 30, 2010, 08:29:04 08:29 »

Gentlemen:

Titi, I was hoping you were right but it just won't see the 3.3 - 3.6 volt signal. I will build an interface  such as dreamcat or Falcon or Hate (Good idea!) suggested and order the level shifter devices next week. I will post the board design in EAGLE for the display (very simple, very neat) I want to use it with a 68hc908qc16 microcontroller to control an Analog Devices DDS device, but until I got it built I was trying to use it in another prototype as a toy to show address and data for debugging.
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DreamCat
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« Reply #8 on: May 31, 2010, 04:03:16 16:03 »

I remember there is two article about level shifting ong END IDEA....

I searched my harddisk.. here is one.

for my self, I'm using BS170 to done this.

« Last Edit: May 31, 2010, 04:05:31 16:05 by DreamCat » Logged

May be I expressed the wrong meaning, sorry for my bad english. Please correct it for me if you can.
TomJackson69
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« Reply #9 on: June 02, 2010, 12:29:39 00:29 »

How about using just one CD4049 or CD4050 gate for level shifting.

Quote from the data sheet:
“The CD4049UB and CD4050B devices are inverting and
non-inverting hex buffers, respectively, and feature logiclevel
conversion using only one supply voltage (VCC). The
input-signal high level (VIH) can exceed the VCC supply
voltage when these devices are used for logic-level
conversions. These devices are intended for use as CMOS
to DTL/TTL converters and can drive directly two DTL/TTL
loads. (VCC = 5V, VOL ≤0.4V, and IOL ≥3.3mA.)”

It can be used for:
• CMOS to DTL/TTL Hex Converter
• CMOS Current “Sink” or “Source” Driver
• CMOS High-To-Low Logic Level Converter

Please refer to the data sheet to make sure.

Tom
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