On 2013-04-07 17:42, Julian Brooks wrote:
Thanks Martin, really useful stuff.
I've got i2cdetect on the RPi which is how I knew that [gpio] was
setting hi & lo. And good to hear you'll be wrestling with this on the
Pi as well.
In some ways this is good news as we've setup everything from the
'instructables' page already and now just need to get the bloody thing
going (have to to sort the housings out).
Another possible issue is that from my reading it seems that the RPi
doesn't do 'clock-stretching', though I have found a link where they
slow the i2c bus down.
http://www.hobbytronics.co.uk/raspberry-pi-i2c-clock-stretching
Another one here too:
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=44&t=34734&p=294297&hilit=i2c+gpio+direction#p294297
That's interesting as they talk about setting up more GPIO pins for i2c
to run 2 sensors.
Not being able to change the sensors address is a real pain though, as
one of the things that I keep reading about i2c is it's ability to run
up to 128 sensors on the same line - kinda defeats the object! Must be
a way round it.
Maybe there's a way to program the address but so far its a secret known
only to Omron.
"You can use the 5V from the GPIO header on the pi. From the schematic
pin 2 is 5V. Ground is on pin 6. Pin 3 is the i2c data and pin 5 is the
clock. Pullup resistors are already installed on those lines."
Yes, found a good diagram for the GPIO schematic.
http://elinux.org/RPi_Low-level_peripherals#GPIO_hardware_hacking
My understanding was that what we can't do is send data from the sensor
at 5v back into the RPi at 3.5v and it's there that we need to drop the
voltage back to 3.5. Noticeably though the 'instructables' link says
they just did it anyway and was fine (with a disclaimer attached on to it).
The schematic for the RPi shows the resistors already installed, you
don't need to add any. The resistors pull the bus voltage up to 3.3V
when nothing is driving it to ground. Nobody is sending 5V signals, the
i2c bus is either driven (clamped to 0V) or not driven, in which case
the resistors bring the voltage to 3.3V, which is high enough for the 5V
sensor inputs to read as 1.
We got some 4.7k resistors as you recommended - do we only need these
before the sensors? The pdf from digikey has a diagram with a voltage
transformer that we've been presuming is what we need to do?? Presumably
if we put more resistors next to the Pi then we wont have enough voltage
to lift the pin high (many ???). There's also lots of code (C?) on that
pdf, anything you've made use of?
Yes that's what I got the crc calculation from, but as I said it doesn't
give me the right result :(
This is the little add-on board
http://adafruit.com/products/757
I did read it's doable with mos-fet but seemed like another layer where
we can screw-up so have taken the simple option.
Yes you don't need any level shifter, the pullup resistors are enough,
both 3.3V and 5V systems use the same voltage for 0.
Martin
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