Nice!
I was in fact considering the DB series for the new design. They have a lot of peripherals.

Don't forget that the new AVR DB series also has the ability to run on both 3V and 5V at the same time.

One of the ports can run on 3V, with the chip on 5V, and the level conversion is done for you, inside the micro.

JC

Try putting a diode (or two) in between the output of the GPS and the input to the micro (cathode to the GPS, anode to the micro). This should raise the input voltage on the micro (with a pull up) enough to let it work.

Better try simplest level shifter on that one line. There is toons of similar schematics. Search for "level shifter" or "level converter"
[img:cb4b7e60d3]https://www.mcselec.com/userpix/22760_shifter_1.jpg[/img:cb4b7e60d3]

Hi,

I did try week pull ups and pull downs. The only thing that solved the problem was using the same power line (3.3V).
Agree that mixing logic leves is nor a good idea. More and more peripherals are becoming 3.3v now. I will change my boards to use only 3.3v.
The only downside I see is that the RS232 and RS485 drivers and the RTC's are much more expensive in their 3.3V versions.

Thanks all for the answers. Appreciated.

JB

Those are characteristics for Mega324.
0.6Vcc for the 5V power supply not means 0.6V It means 5V * 0.6 = [b:f26dff7c2e]3V[/b:f26dff7c2e]

[img:f26dff7c2e]https://www.mcselec.com/userpix/22760_dc324_1.jpg[/img:f26dff7c2e]

Some modules have only Open Collector so some pullup must be done but you cant use internal from the Mega because 5V.
You can try pullup it into 3V3.

Logic levels is probably your problem. Even though the data sheet says that at Vcc of 5v, 2.6v is treated as logic high and 2.2 is a low, Im wondering if your GPS output is outside this range. Have you tried a weak pullup or pulldown on the mega input that is looking at the GPS signal? What does the pulse look like on a scope?
It is a simple matter to use a FET as a logic level shifter. It is bad practice to mix logic levels.

Hi All,

Thanks for all the awnsers.
The 1PPS pulse is 100ms.

But, seems that the proble is electronic and not software related.


I have the system powered from 5V and the GPS module is 3.3V, so I have an internal 3.3v regulator to power the GPS
Since the ATMEGA can take logic levels of 3.3V I dind't see a problem. In fact I have several boards that use this same setup (but don't use interrupts) that work just fine.

If the system is powered from 3.3V (atmega and GPS, it works).
I don't understand why. Somebody that knows more electronics than I do please explain...

Thanks

How long is the 1PPS pulse of yor GPS Device? At least 50ns is needed for MEGA324A. Maybe your optocoupler works because the pulse is stretched by this relatively slow device.

Here is the code I am using to count rain pulses from a rain gauge and wind speed pulses from an anemometer. The wind pulses are 4.8ms wide and can occur several times pe second.
Rain pulse is 20 mS and only x1 every 1 min or so in heavy rain.
The processor is the atmega1284p



[code:1:8704c53005]

'Initialize interrupts for wind input on pin A.4 (PA4, PC Int0)
' rain on PCI3 wind is on PCI4

pcmsk0 = &B00011000 'enable PCI3(rain) and PCI4(wind speed pulse)
PCICR = &B00000001 'enable PC Int0
on PCint0 count_anemometer saveall 'no brackets - must look like a label


'***********************************************
sub count_anemometer()


'rainswitch = tipping bucket rain gauge
if rainswitch = 0 then

if dn15 = 0 then

rainsng = rainsng + rain_bucketsize
Totalrainsng = Totalrainsng + rain_bucketsize 'rain in last 24 hrs
print #1, "rain";Totalrainsng
end if

dn15 = 1

else
dn15 = 0
end if

'windswitch = anemometer lo going, normally hi
if windswitch = 0 then

if dn16 = 0 then
incr windpulsecountw
Toggle Wind_led
end if
dn16 = 1

else
dn16 = 0
end if


end sub
'***************************************

[/code:1:8704c53005]