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Jim90
Joined: 27 Apr 2013 Posts: 55
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Voltage supply |
Posted: Fri Jan 24, 2014 5:41 pm |
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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 ? |
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temtronic
Joined: 01 Jul 2010 Posts: 9170 Location: Greensville,Ontario
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Posted: Fri Jan 24, 2014 6:24 pm |
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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 |
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bkamen
Joined: 07 Jan 2004 Posts: 1611 Location: Central Illinois, USA
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Posted: Fri Jan 24, 2014 10:20 pm |
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I second that...
And be SURE that your sensors are strictly 5V, they may not be.
Datasheets are your friend.
-Ben _________________ Dazed and confused? I don't think so. Just "plain lost" will do. :D |
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Gabriel
Joined: 03 Aug 2009 Posts: 1067 Location: Panama
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Posted: Sun Jan 26, 2014 1:50 pm |
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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. _________________ CCS PCM 5.078 & CCS PCH 5.093 |
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asmallri
Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia
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Posted: Sun Jan 26, 2014 6:03 pm |
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Gabriel wrote: | 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. |
Analog sampling accuracy, repeatability and reference voltages will influence the choice of power supply design. A simple charge pump approach can and often does produce an unacceptable level of analog noise on the analog power rails.
There are lots of different ways of designing the power supply section. It is not uncommon to have three regulators stages even when the analog and digital logic are both powered from the same nominal voltage. Say you example you have a nominal input supply of 12 to 18 volts DC and require 5 volt for the analog section and 3.3 volt for the digital logic. The first stage switching regulator could accept the 12 to 18 volt input and produce the intermediary 5.5 volt output. This 5.5 volt then feeds a 3.3 volt switching regulator to drive the PIC and a 5 volt LDO regulator to drive the analog section. _________________ Regards, Andrew
http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!! |
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Gabriel
Joined: 03 Aug 2009 Posts: 1067 Location: Panama
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Posted: Mon Jan 27, 2014 6:36 am |
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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 |
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Jim90
Joined: 27 Apr 2013 Posts: 55
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Voltage supply |
Posted: Mon Jan 27, 2014 6:04 pm |
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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 ? |
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PCM programmer
Joined: 06 Sep 2003 Posts: 21708
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Posted: Mon Jan 27, 2014 7:29 pm |
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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. |
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