mcf51jm128 5v tolerant inputs

[mauricio2346]

Hi.

i been working on a design based on mcf51jm128.  i added dual power supply, so the microcontroller can be powered with 5v and 3.3v.  also, there're some peripherals like MicroSD (3.3v), CAN transceiver and RS845 transceiver. (5v). 

i was wondering if JM128 pins are 5v tolerant, so i can use CAN and 485 modules with 3.3v on ucontroller safetly.  

it could be possible to use a series resistor between both devices without burning peripherals or microcontroller?

the schematic is still on process, so there're missing parts and things to change

thanks;

Mauricio

[celsoken]

Dear Mauricio,

I've been involved in a similar design almost 2 years ago.

I chose 5V-CAN xvers because the 3.3V ones did not meet full specs and they were also very expensive.

TXCAN: I've lowered 5V supply voltage so I could drive directly with 3V3 - or you can use a single port HCT device to properly do that without changing voltages.

RXCAN: You can use a level converter IC or the same HCT port fed by 3V3 with the series resistor at the input. Of course you could use the MCF51 input clamp diodes and limit this current lo less than your current consumption but I'd rather use the HCT input clamp diode to avoid current overload at MCF51 power pins.

I hope it helps,

Celso

[kef]

There must be injection current limits specified somewhere in datasheet. You should use current limiting resistors to keep injection currents within limits.

But best you can do would be to use 3.3V CAN and RS485 transceivers. Look for example at  SN65HVD232 for 3.3V CAN transceiver. It shouldn't be hard to find of 3.3V RS485 transceiver.

[mauricio2346]

hi kef.  unfortunately, those 3.3v devices such as 485  and CAN tranceivers are hard to find here in Colombia.  i've in this  moment a smd SN75176, so i want to use it.  found an option yesterday from sparkfun.  using some resistors and a diode to conect 5v devices to 3.3v.  (as you can see on attachments). 

i've tested this circuit and works fine, but i've a question. if i connect the JM128 to 5V instead of 3.3v, this level converter could work?

im a little concerned about diode behavior. 

thanks;

Mauricio

[JimDon]

All you really get is this:

Input voltage VIn–0.3 to VDD + 0.3 V

So if you are running on 5V, it is 5V tolerant. If you are running on 3.3 it is not.

Basically, you are going to have to decide what supply you are going to use for the MCU and then design the interface to the drivers accordingly.