low serout voltage to 3.3v

[noycesd]

Hi Guys,

I am building a data logger which is working fine except that the data is sent serially to a spark fun logomatic unit. I have to run the Picaxe at 5v due to the clock and eeprom chips used but need to drop the serout on pin 7 to 3.3 volts to work with the logomatic. Any advise appreciated. Si

 

[Dippy]

Have you thought of level shifting using resistors in a simple potential divider arrangement?

I don't know what the electrical spec of that device is but I have successfully used 1K5/3K3 and 1K8/3K3 to go from PIC to 3.3V SD card.

 

[westaust55]

How many signals do you have to transfer? If serial may be only 1 or 2.

I am about to do something similar from 5Vdc on PIAXE to 3V for graphic LCD using a LP2950 voltage reg and an 74HC245 to convert signals, but I will have 5 signals to convert.

Have a look at the attachments to post 6 on
http://www.picaxeforum.co.uk/showthread.php?t=10014

or a commercial version of the same thing . . .
http://www.nkcelectronics.com/5v-to-33v-logic-level-shifter-bo.html

 

[westaust55]

Sparkfun themselves have a tutorial at:
http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=65
:

 

[noycesd]

a divider would be the simplest route, how would this connect up? do i need to connect to gnd?

si

 

[Dippy]

SerialOut >>> 1k8 >>(to your thing here)>>>3k3>>>Gnd.

Not exactly 3.3V but a close shave.

Read up on resistors in series and learn the simple calculations. It'll save you a lot of time. Well, actualy it won't, but you may learn for the future.

 

[noycesd]

cheers Dippy.

Will nip out for the required values and give it a try.....this will be the end of a whole heap of trouble if it works.

 

[Dippy]

Get yourself a boxfull of resistors while you're at it.

 

[westaust55]

Interfacing 3V and 5V devices

Bit more than is needed in the case mentioned in post 1 but for the record
here are some links to applications notes and datasheets on the subject of 3V (i.e. 2.6 to 3.6V) and 5V device interfacing.

Interfacing 3V and 5V applications:
http://www.semiconductors.philips.com/acrobat_download/applicationnotes/AN240.pdf

Voltage Level Conversion:
http://focus.ti.com/lit/an/swra071/swra071.pdf
=========================================
5V to 3.3V and back from 3.3V to 5V
(Works down to 1.5V)

74LVC4245A Octal dual supply translating transceiver; 3-state
http://www.semiconductors.philips.com/acrobat_download/datasheets/74LVC4245A_5.pdf

MAX3370/MAX3371 logic level translators
http://pdfserv.maxim-ic.com/en/ds/MAX3370-MAX3371.pdf

More level translators from Maxim:
http://para.maxim-ic.com/cache/en/results/5043.html
================================
5V to 3.3V
Inputs can be driven from either 3.3 or 5 V devices.
(The 74LVC125A and 74LVC244A will not replace the 74LVC4245A or the MAX3370/MAX337).

74LVC125A:
http://www.semiconductors.philips.com/acrobat_download/datasheets/74LVC125A_4.pdf

74LVC244A:
http://www.semiconductors.philips.com/acrobat_download/datasheets/74LVC_LVCH244A_4.pdf

==================
and a whole stack of tips and tricks by Microchip
http://www.newark.com/pdfs/techarticles/microchip/3_3vto5vAnalogTipsnTricksBrchr.pdf