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Re: I2C & Power supply



Hi,
See my answers below...

----- Original Message ----- 
From: "Peter Mueller" <peter.mueller.ls@xxxxxxx.com>
To: "Axis mailing list" <dev-etrax@xxxxxxx.com>
Sent: Monday, June 02, 2003 11:16 AM
Subject: I2C & Power supply


> Hello!
>
> I got an Axis developer board LX last week and start to
> explore it.
>
> Two questions came up to which I couldnt find answers
> in the online doc and the list archive:
>
>
> I2C:
>
> I found several answers concerning the I2C bus and the
> possibilities to use it in this list, but most of them
> seem to discuss the predesessor cpu without MMU and eLinux.
>
> Am I right with this:
>
> .) The Etrax 100LX has two pins dedicated to I2C

Those pins are actually part if the Gerneral I/O port PB
but can be configured to be accessed through special registers
seperately from the other pins on PB.
Depending on kernel config the driver either uses the special registers
or the normal PB registers.

> .) The protocol is implemented in software (in the
>     I2C modules (drivers)
Yes.

> .) These two pins are not officially reachable on
>     the Axis Developer Board LX but they would be
>     available at the pins of the RTC chip.
Yes - or the eeprom.

> .) There is another IO pin used to switch between
>     the serial EEPROM (I2C conformant) and the RTC chip
>     (using a simple non I2C protocol)
Yes, the /RST signal to the RTC  (/I2C_SELECT) is used to disable the I2C
clock to the eeprom
when the RTC is accessed.

> .) The RTC chip is of not much use since there is no
>     battery to keep the time over power cycles.

It's backed up by a capacitor if the trickle charger in the RTC is enabled
(can be made with a kernel config option in later kernels or from
user mode with the hwtestrtc utility)
With the capacitor the board handles short power outages without loosing the
date and time.
If you're handy you can replace the capacitor with a real battery - the PCB
is prepared for it
(the three holes in a triangle around the capacitor)

> So if I manage to grab the three signals at the RTC chip,
> and connect my I2C components with a simple logic that
> uses the switch signal to mask off the communication
> with the RTC chip, could the I2C implementation be used
> with the Developer board and the Linux 2.4.19 kernel
> delivered with the board ?

Probably better to take the signals at the eeprom.
Modulo bugs in the i2c driver it should work
Perhaps the timing needs to be changed.

 > Are there any other things that hinder the use of
> the I2C interface ?
>
> Is there an alternative way to connect I2C devices
> allready explored and impelemented by someone ?


>
> Power supply:
>
> My device should be battery powered.
>
> I found in the schematics that there should be
> a point on the developer board where DC power can be
> supplied as an alterantive to the AC power supply
> delivered.
>
> Which Voltage should be suppiled and where to connect
> the battery to ?

You can connect the DC power directly to the AC inputs,
either on the standard connector or on the green terminal block
pin 1 and 2.
The voltage will be slighly reduced by the rectifier bridge but that
shouldn't be any problem.
Otherwise the + can be connected to pin 10 of X25.

12V is what we recommend but it can be down to 5V I think,
but that would be on the edge so please test and measure before
deploying it, 9V would probably be just fine.
24V is the absolute maximum but that is on the edge of being to much.

> What average power consumption would I have to expect?
About 3-4 Watt if I remember correctly.

>
> Thanks in advance for any ansewers,
> Peter
> Elimpex
>
>
> -- 
> Peter Mueller  Elimpex - Medizintechnik GesmbH.
>
> Tel: +43-2236-41045-0   Fax: +43-2236-41045-9
> mailto:peter.mueller@xxxxxxx.com http://www.elimpex.com/

/Johan