5V Micro to 3.3 Volt DSPIC conversion

[Angy]

I'm looking for logic level shifters to convert a 5volt microcontroller board to 3.3 Volt dspic processor.
I'm going to use the same board that I designed for the 5v microcontroller but I will need a level shifter or some buffer to convert the 5V to 3.3 Volts for inputs to the dspic and outputs from the dspic need 3.3 volts to 5 volts to interface the board that I already designed.Any sugestions?

 

[blueroomelectronics]

Can you use a dsPIC30F, they are 5V rated.

 

[Angy]

I'm looking to use the 100pin version of the dspic due to all the functions I need to design on the project. And dspic33f is newer and has the pins I need for the design.

 

[DirtyLude]

Without looking at any datasheets and having no experience with a dsPIC, I would check to see if the dsPIC IO lines are 5V tolerant. Every 3.3V uC I've worked with can take 5V IO signals.

 

[dknguyen]

THe digital inputs on the dsPIC33F are tolerant of 5V inputs. THey cannot necessarily drive a 5V digital line of another IC though. You have to check the datasheet of the other device to see that the logic input high voltage threshold (Vih) is 2V (or less). The logic low threshold is not a problem.

THis is NOT TRUE of the analog inputs (ie. the ADC). If the input is set to analog and you apply 5V to it, it will be damaged.

 

[DirtyLude]

I do A/D conversion on the board and as you said I will not be able to apply 5v to the dspic analog input channel cause it will be damaged.

You are asking for a way to scale down your analog inputs for the 3.3V A/D. That is a totally different question than asking about 5V<>3.3V level shifters. If you don't give us this information, nobody can help you.

Right now I have a MC14504B hex level shifter on the output side of the micro and looking into the datasheet its logic high is 5v.Is there a way to use the same for dspic?

Logic level high on the input side of the MC14504B is not 5V. It's different based on the supply voltage and the MODE pin. Based on the datasheet, if you set the MODE to TTL level inputs, 2V is your input logic high, so it sounds like that would work, for your digital outputs but I have no experience with it, so maybe someone else will correct me.

 

[arhi]

3v tips and tricks from microchip: https://www.electro-tech-online.com/custompdfs/2009/02/en026368.pdf
**broken link removed**

 

[shingadaddy]

Analog = resistors.
Digital = 74LVC07.

 

[Angy]

I understood dividing resistors could be used to scale input to analog.
Reading the data sheet of 74LVC07, i got a little confused. So you are actually asking me to use this chip for interfacing the inputs and outputs of dspic to 5v logic?

Their supply voltage 1.65 to 5v, so if i have vcc=5v and 3.3v input then their output could be 5v?

 

[dknguyen]

To scale down 5V analog to 3.3V analog for your ADC, use a resistive divider and op-amp buffer.

YOu do not need the 74LVC07 for your digital inputs because the 3.3V dsPIC can tolerate 5V digital inputs. But check the input logic high threshold on your other devices to make sure the 3.3V output of the dsPIC can communicate with them properly. If it cannot, they you will need something extra for these pins (the 74LVC07 for example). But it's far more convenient to pick 5V devices that have 3.3V logic high thresholds than to add extra circuitry.

 

[Angy]

Thanks a lot for your suggestions.

 

[shingadaddy]

From the 74lvc07 data sheet:

Inputs can be driven from 1.8-V, 2.5-V, 3.3-V (LVTTL), or 5-V (CMOS) devices. This feature allows the use of
this device as translators in a mixed-system environment.

The output is open drain and can be pulled up to whatever you need, up to 6.5 volts.If you are in a pinch and need it, it works good but always best to check datasheets to see if the devices you are going to use will play nice together.

 

[Angy]

I'm using LVC07A chip to work with mixed signals of 3.3v and 5v on a dspic board.
According to the chip's datasheet the outputs can go from 0 to VCC in the active mode so I supplied VCC to the CHIP around 5v, since I want a 5V output. My input on pin no 1 is 3.3v (from o/p of dspic) but I dont see any output on pin no2 of the chip.

Can someone help me plss?

 

[Angy]

Texas instruments have suggested this LVC07A is not useful for level translating.

 

[Wp100]

Hi,

Think you really need to try one of the chips as I mentioned in my recent post 'Unused Gates'

These are Dual Supply ( eg 5v and 3v3 ) 8 or 4 bit Bi-Directional translator chips, everything you would need in one simple package.

They seem to be only smd but are hand solderable with care.

They are stocked by Farnells in the uk TEXAS INSTRUMENTS|TXB0108PWR|TRANSLATOR, 8BIT, BIDIRECTIONAL | Farnell United Kingdom

Mouser also have a very similar chip SN74LVC8T245PWR

 

[AtomSoft]

Dude i use simple SN74LS245N ( $0.80USD):

SN74LS245N

Mainly because i own:
https://www.futurlec.com/Mini_Logic.shtml

and it works well so i bought a few of the same type of IC's aka 74ls245

 

[Wp100]

Dude i use simple SN74LS245N ( $0.80USD):

You will have to help me there AtomSoft - the chip runs at 5v, so with a 5v pic on one side how do you get 3v3 DSPic on the other ?

 

[AtomSoft]

heh sure sorry bout that:

74LVC245A or 74LVCH245A = 2.7v - 3.6 eith 5v tolerant input

$0.25 USD
74LVC245 - Octal Bus Transceiver (20 pin SOIC)

74LVC Series

DATASHEET:
74 Series - 74LVC SMD - 74LVC245A