How to interface 5v MCU with 3.3v CMOS device?

[a4lm]

Hi,

How(if) can I use 74hct245 to interface 5v MCU with cmos(3.3v) device(gps module)?
I've tried using it directly in-between but had no luck.

 

[fouwad]

Re: Interfacing TTL-CMOS

you will have to use an intermediate voltage level ie MAX232 for ur microcontroller and an equivalent voltage converter on ur gps module
its not an issue ull find alot of material on this
but NEVER ever try to connect them directly nxt time or else ull end up in burned circuitery

 

[a4lm]

Re: Interfacing TTL-CMOS

Thanks for the reply, yes I burned one already..
So 74HCT245 can't be used for this, even if one-way communication is enough?

 

[cubanflyer]

Re: Interfacing TTL-CMOS

Is this thread of any use.

https://www.edaboard.com/threads/11539/

 

[alexan_e]

Re: Interfacing TTL-CMOS

You can use the 74AHC series (74AHC245) to interface the different voltages.
The AHC series can tolerate an input of 5v with a power supply of 3.3v so it can take 5v as input and output 3.3v
or in the reverse direction it will have 3.3v as input and 3.3 as output

http://ics.nxp.com/products/ahc/datasheet/74ahc245.74ahct245.pdf

Alex

---------- Post added at 13:51 ---------- Previous post was at 13:44 ----------

74LVC245 can be also used as mentioned in the above thread link, it can also tolerate a 5v input with 3.3 power supply.



Alex

 

[fouwad]

Re: Interfacing TTL-CMOS

if ur able to make a proper interface between cmos and ttl then u can make bidirectional communicaiton

 

[FvM]

Re: Interfacing TTL-CMOS

To supplement Alex' suggestions, I think, it's necessary to analyze both directions separately:

5V_CMOS -> 3.3V_LVCMOS
the voltage must be reduced to keep the input rating of the 3.3V side. 5V tolerant 3.3V logic, as 74LVC supplied by the 3.3V side would be the solution for high speed, e.g. clock or data bus. For a serial interface, a simple voltage divider, or a series resistor with a schottky clamp diode is sufficient in my opinion.

3.3V_LVCMOS -> 5V_CMOS
I assume, that the 5V side has a standard CMOS specification, needing a high level 0f 0.7*VCC = 3.5V. Some processors have however a TTL input specicification and can be driven by 3.3V_LVCMOS directly. To increase the level to 5V, a level converter is required. Standard 74HCT logic is a widely available solution. (The "T" stands for TTL level compatibility, Vih,min = 2.0 V)

 

[rmarkespanto]

some one please give me the connection between my 5v mcu to 3.3v serial camera.... and my mcu to gsm/gprs modem sim900d... im using pic18f4550?...?? someone help me please...:idea:

 

[arbj]

you can use an appropriate resistor divider combination to bring down the voltage from 5 V to 3.3 V

A 2K and a 3K resistor divider combination works very well.

To convert from 3 V to 5 V, a NPN transistor could be used, in this case the states will be reversed, ie: a high of 3.3 V line will be become low at the 5 V stage. So you will have to reverse the logic states at the code level, also there may be a restriction on the pulse rates.

A better solution is to connect the 3 V output line directly to a TTL buffer port of the PIC. In the case of the 18F4550, refer to the datasheet (section 28.3, page 374, Input High Voltage),TTL buffers can detect levels of 2 V. So simply connect the 3 V output line to the TTL buffer port of the PIC, refer to the datasheet and see which of the ports have a TTL buffer (marked as TTL).

You could also use the PIC18LF4550 instead and operate the entire system at 3.3 V and save all the trouble.

thanks
a

 

[rmarkespanto]

can you please send me the schematic sir... this is my circuit design, im just attach it... im using proteus sir???