just need some clarification on this datasheet.

[tony_g]

one of my next projects involves using a real time clock and an spi wireless module.

i will be running the project at 5v as from what i understand with the DS1307 is that it does not like less than 4v5 for the i2c comms to keep active.
however my rf module has a max Vin of 3.6v but from a look at the datasheet it says max vin on any digital input to be no more than 5v25, and digital input i would assume means the MOSI and CLK pins from the 40x2.

would i be correct in taking this to possibly mean the data pins are 5v tolerant?.
its not an issue if it would be safer to run it at say 3v3 which is below the max Vin but if that is the case then what option should i approach for the level conversion between the 40x2 and the rf inputs,presumably something similar to the level shifter that gets used with 5v mcu and 3v3 i2c.

initially i was going to power the project at 3v3 which would have worked for the rf module and have to admit i assumed the rtc chip would be fine with that voltage but after reading around a bit it seems it needs more juice to keep it happy.

to date i have not done any level shifting for devices communicating at different voltages so any reference diagrams or suggestions for parts i need to add to my end of month components splurge would be greatly appreciated,even better if i can keep it along the smd route as with the other board components.

thanks,
tony

View attachment DS-RF2400-NANO-V10-EN.pdf

 

[tony_g]

hmmm having another blonde day i think, im assuming i can just use the i2c level converter circuit with the fets and run the 40x2 and rf module at 3v3 and the rtc at 5v into the level shifter to the mcu, its so easy to overlook things and make them more complicated, d'oh

 

[nick12ab]

That certainly suggests that the level shifters are built in, however the datasheet recommends that 100 ohm resistors are used on the signal lines.

 

[westaust55]

Another option would be to consider the DS1338 with it compatible with the DS1307.
It have versions for typical operating voltages of 1.8V, 3.0V and 3.3V.
Another advantage is that it will operate with the i2cFAST parameter.

It only comes in SOIC package so an adapter may be required.

 

[tony_g]

i saw that too but then saw the max Vin specs for the data pins, im still waiting for them to arrive and just want to make sure i dont inadvertantly kill them by assuming lol.
i suppose i should send an email to the supplier to verify with them, i just hope the chinglish translation in both directions is not too bad.

 

[tony_g]

thanks nick, and westy your post just popped on while i was typing a reply but i shall look at that option as well thanks

 

[Paix]

Is the use of 3.1V (or 3.3V) zener protection diodes and a 100 ohm resistor good for interoperating between 5V and 3.3V logic?

 

[MartinM57]

...OK, if you're happy with a rather large 19mA down the zener, which has to come from the PICAXE pin and ultimately the power supply.

 

[tony_g]

well i guess i shall look towards the DS1338, it will be easier to be able to keep it all with the same supply voltage and not chance killing the rf modules.
as cheap as they are, they just take too long to arrive so i have to try and keep them working until i can get some more lol.

thanks for the input and suggestions guys.

 

[Goeytex]

Here is a simple circuit that does bidirectional level shifting and works well with I2C. It is from: Phillips Application Note AN97055 . I simulated it in SPICE and it works well.

 

[tony_g]

thanks goeytex, i think i have the phillips i2c doc somewhere in the computer, time to clean up the files and find some valuable pdf's again i think lol

 

[westaust55]

Is the use of 3.1V (or 3.3V) zener protection diodes and a 100 ohm resistor good for interoperating between 5V and 3.3V logic?
View attachment 12871

Over and above the high current mention by Goeytex, whether using a resistor and Zener or two resistors as a potential divider, such arrangement can o ly reduce the signal voltage level. For bi-directional signals, such as the i2c data line, the device connected to the higher supply voltage will not receive a signal with a sufficiently high voltage for a high (1) state to be recognized.

 

[inglewoodpete]

If it was my project, I'd be using the MOSFET (TO-92 or SOT-23) solution described in the Phillips Application Note.

 

[tony_g]

i think i will probably look at using the DS1338 soic package as it will allow me to keep to the one voltage for all devices connected to/using the circuits power so everything is kept happy powerwise.