Voltage supply

Jim90 · Joined: 27 Apr 2013 Posts: 55

I will need to ask, if i use a microcontroller of power supply of 3.3v and sensors of 5V. If i use 2 different supply is it ok ?

temtronic · Posted: Fri Jan 24, 2014 6:24 pm

Yes and no....

You WILL need to add some kind of 'logic level converter' between the devices. What you use depends on the sensors and interface of the PIC.
If they are all I2C devices, you 'might' get by with some discrete mosfets and resistors. More than 2 lines, you should look at an IC for that purpose.

Most PICs have a 5 volt 'brother' designated 'F'. 'LF' versions are low voltage(aka 3 volt) devices.

Without knowing what 'sensors' and which PIC, I can't say which is better, but having both PIC and sensors of the same voltage makes the hardware a LOT easier. Less chance for wiring errors, lower power consumption, etc.

hth
jay

bkamen · Posted: Fri Jan 24, 2014 10:20 pm

I second that...

And be SURE that your sensors are strictly 5V, they may not be.

Datasheets are your friend.

-Ben
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Dazed and confused? I don't think so. Just "plain lost" will do. :D

Gabriel · Joined: 03 Aug 2009 Posts: 1067 Location: Panama

You can generate a higher voltage from the 3.3v with a charge pump.
In some cases like the LM35, the max temperature/voltage output is 1.55v or 155C.

So for THAT particular sensor, you can read it directly regardless of it being powered by 5v.

G.
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CCS PCM 5.078 & CCS PCH 5.093

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

Gabriel · Joined: 03 Aug 2009 Posts: 1067 Location: Panama

I agree with many of your points.
I was simply offering options.

I dont know if i agree with not being able to generate a proper rail with a charge pump. Obviously ripple/frequency and the load have to be properly balanced, but its not imposible to get "good" performance. IMO a switching regulator could produce similar noise levels.

I recently developed a self balancing -V/V+ supply for a buffer op amp & phase inverter... ( -V 2 stage charge pump with feedback loop) it works incredibly well (IMO)... its a battery operated system and i get the -/+ rails to .01(accoding to my o-scope) from each other even when loaded.
Thats better precision than the FVR on the 16F1825 im using.

All im saying is that its an option.

G.
_________________
CCS PCM 5.078 & CCS PCH 5.093

Jim90 · Joined: 27 Apr 2013 Posts: 55

I want to use a microcontroller from the dspic33 series (which need 3.3V supply) and a ds1307 sensor (which need 5V supply). What is the best choice ?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Regarding the i2c bus connections between the dsPIC and the ds1307:
The ds1307 data sheet says Vih is 2.2v minimum. So you could use it
with a 3.3v PIC (without level translators). You can use 3.3K pullups on
the SDA and SCL lines. Connect the pullups to +3.3v. The Vdd of the
DS1307 must go to +5v.