PIC24(+3.3V operated) interfacing with other +5V devices

[arup]

hello,

I am planning to use few +5V devices(which were already used in my older project) in my new project with PIC24 chip.My project is based on PIC24FJ128GA110.I want to drive certain +5V operated devices like MUX - hef4067 , Shift register - 74hc595 etc...Can I use Digital i/os of pic24 (+3.3 V micro) to directly drive the control pins of those +5V operated devices ??What I understand is to use those pins i need to ensure following:

a) Those pins need to be configured as opendrain in firmware.They will be l pulled up by external resistors to +5v
b) These pins need to be any digital only pin .They shouldn't be ANx pin.

Pls correct me if i m wrong and also suggest if other precautions has to be taken.

What are the general consideration where 3.3 V operated Micro has to be ineterfaced with +5V operated devices in the Application

Thanks & Regards,
Arup

 

[cdh7]

Hi Arup,

a) Those pins need to be configured as opendrain in firmware.They will be l pulled up by external resistors to +5v
b) These pins need to be any digital only pin .They shouldn't be ANx pin.

That's exactly right. And the PIC24 also tolerates +5V inputs on certain pins in case you need to read data from your 5V peripherals.

Keep in mind that the digital inputs on your peripherals have a certain capacitance which will form an RC circuit with your pullup resistors. This makes open-drain outputs somewhat slower. If you use 1k pullups and keep your signals less than a few megahertz, I guess you won't have any trouble.

Regards,
Chris

 

[FvM]

Using 3.3V compatible 74HCT is possibly a better solution, particularly for higher signal speeds. It also saves the energy for the pull-ups.

 

[xaccto]

I have same question with a dsPIC33 circuit (3.3V)
i'm thinking of connecting a MAX232 which runs on 5V to connect to dsPIC33 pins.
I suppose a resistor for MAX232 out to dsPIC of maybe 1k?, inputs should be able to directly connect, or need a level converter (transistor? )
don't want to risk the dsPIC chip so just checking with the forum braintrust
first.......

 

[GSM Man]

Microchip has some great documentation that addresses these issues on their web site:
3V Tips 'n Tricks: https://ww1.microchip.com/downloads/en/DeviceDoc/41285A.pdf
3V Newsletter: **broken link removed**
Don't pay for level translators in systems using multiple power-supply voltages**broken link removed**
3V Design Center:**broken link removed**

 

[FvM]

It would be a good idea to take a look at datasheets. MAX232 have a TTL input level specification and can interface with 3.3V CMOS
logic outputs without needing level translation or open drain drivers.

 

[ZASto]

Better, yet, MAX3232, the chip is designed to work with 3V levels.

 

[FvM]

Better, yet, MAX3232

Better, if a 5V supply can be saved this way.

 

[ZASto]

MAX3232 can, and will operate on 3V supply (I'm running one on 2.7V, using it as a level translator - PIC16F877A -> MAX 232 -> MAX3232 -> Telit GM862 GPS)

2.7V obtained by zener/resistor combination