Hello guys i just buy a EM-406a module and i need some help about the interface between GPS and uC.
Typical connection

uC <------------> GPS
--------------------------
Rx <-------------- Tx
Tx --------------> Rx

right right?
But :mad
from the GPS manual :
Electrical level : TTL level,Output voltage level :0-2.85V,RS-232 lvl

so this might be a problem ? since my uC works on 5V for Logical High
right right?

Ok now lets go back to my Electrical Characteristics of my uC
Vih = min:0.2Vcc+0.9 and max:Vcc +0.5. Vcc = 5v
Voh = Vcc-0.3 , Vcc-0.7,Vcc-1.5 *with my meter i get 4.97V but why?
so should i use the typical connection or i damage the module?
i think Vihmin = 1.9V is ok for Rx <---------------- Tx
what about Tx --------------> Rx, Voh=4.97V is too much for the module ... right right?


any explain to my questions or any source for study will be helpfull thanks

*also what is your opinion about my gps module ?

use a voltage divider on tx line..



Vr2 = R2 . V /(R1 + R2)

Vr2 = 2.85V
V = 5V

2.85/5 = R2/(R1+R2)

1+R1/R2 = 5/2.85

1+R1/R2 = 1.75

R1/R2 = 0.75

R1 = 0.75R2

check your inventory and select R1 and R2 according to above equation.

Nice thats the solution that i have in my mind Ajay
so R1 must be >= 0.75 * R2
my resistor invertory is so limit ..
i will use R2 = 261ohm and for R1=216 ohm with respect to above equation

but now another question arised and its about currents
i= 2,74 / 261 = 10,5 mA, should i use bigger resistors?
lets say i connect two 1K resistors in series with R1 and R2
R11=1K+R1 and R22=1K+R2 .

thanks

most gps have two o/ps
TTL and rs232


if you have a rs232 o/p.. then use
gps->max232->uC
and
gps<-max232<-uC
this will be the safest solution.

OK shyam i connect it with max232 also i sent it but accident zero's i think ...

and later i connect it to my pc
for a simple test
pc-max232-gps

but whats wrong now ?? the led is not flashing ...
using SirfDemo i get those things see picture

http://img43.imageshack.us/img43/8335/wtfjnm.jpg

also it works only on 4800 baude rate ,at the other speeds i get errors like comm: ce_break
did i kill it? :-s

SOS SOS SOS HELP MEE!!!:-s

EDIT


OK IT WORKS BUT NOT INSIDE MY HOUSE !! :mad

it would work only on 4800 bps.

is it stable... if yes then you getting actual data.. flashing leds indicate void data.

and please do not shout.

GPS should be placed in open sky to get satellite data. it wont work inside house or under roof or any kind of metallic shielding.

Well i try your ideas guys but they seems dont work ..

1st method using max232

i was getting the same none valid character all time
dunno why! :mad

actually when i was tryin to connect my GPS to PC the max232 was the only way to do it, and SirfDemo was able to get valid Data of GPS.

2nd method using a voltage divider with resistor ratio 0.75

none valid characters ....
dunno why too ! :mad

The only way to get valid data was the typical connection between
uC -- GPS
Rx <-------------- Tx
Tx --------------> Rx

If this connection method works, why not just use that ?.

yeah i will use this one, much simpler and much cheaper ...
but i still wondering whats wrong with the others too.