question about io logic levels

[tramp]

Hi ,
I am using a controller(U1) which has supply voltage (VCC) 3.6v & I/O logic supply voltage 2.75V(vccio). Anather controller(U2) in my circuit is having VCC=5V & i/o logic level on 5v. I have to interface RX & TX pins of UART between them.
Now my question is,
1) for interfacing tx pin of U2(5v io) to rx pin of U1(2.75v io) can I use resistance devider for matching level requirement.
2)for suppling VCCIO to U1 which is 2.75 can I use resistance voltage devider from 3.3V.

Thnx in advance.

 

[gajen]

Use a level shifter from 5v to 2.75 volt interface. Resistor divide will take high current and it will impact speed also. See if RX device can tolerate 5V input signals. ARe these CMOS chips. Look at maximum ratings also.

For supplying 2.75 resistor divider is not right as it has to drive power and there will be current requirements which will cause huge IR drop and lot of switching noise also. Use a separate supply for that

 

[tramp]

thnx gajen for your reply.
yes both r CMOS ics

 

[GrandAlf]

3v O/P to 5V I/P should be ok(just). 5V O/P to 3V I/P Series resistor and 2.7 or 3.3 Zener to pin to ground on 3v side. Better solution would be RS485 or 232 chips.

 

[tramp]

thnx GrandAlf,

But 485/232 will add extra cost.

 

[Iouri]

What is reason to use 2.75VCC IO? Why don't you use 3.3V instead? Creating all those mess like 3.3V 2,75V 5V adding cost for the unit and also headeches for the person who has to make it work... Try to use same voltage across the board, otherwise look into the TI 74LHS245

Good luck

 

[tramp]

Hi Iouri,

2.75VCC IO & (3.3 to 4V) VCC are the specification for U1 & (4.5 to 5.5V )VCC
is specification for U2.

TRAMP.

 

[scart]

i think u beter use a buffer chip like 74lh244 or use max232
i think 74lh244 is faster
and u could use a resistors divider for supply voltage cuse its not draining high current and there is no bige power dispathion in the resistors
regards

 

[kieennx]

Hi tramp,
you can go to website of Texas Instrument (www.ti.com) to see how they interface their low-voltage DSPs (3.3V) with TTL inputs/outputs.
Hope this help.
Kieennx.