Beginner's queriesss on I2C Serial Connection

Interesting idea Nick.
Not sure how the active transmittter will like having 2 unpowered loads on the line.
I would suggest instead adding a demux chip so that only the selected device sees the clock line toggling. Requires 2 more arduino pins.
Use part like 74LS138.
With G1 high, G2B/ low, the selected output will mimic the state of G1A/.

Should be fast too - just 13/14 nS of delay from G1A/ switching to the output switching.

The part has an Off mode, attained by taking AVDD and DVDD both low.

AVDD has a 3.3V Max limit, DVDD has a 2V Max limit.

"Off Mode
During off mode device is off. No device functionality exists. Both AVDD and DVDD are low. The I2C bus is enabled for
use by other devices on the network in off mode. In this mode the I2C pins shall be in a high impedance state."

"Power Management
This device is capable of operating with a single supply (AVDD) or dual supplies (AVDD and DVDD). Pin VREN makes
this selection by enabling the internal digital supply voltage regulator. When VREN is tied to AVDD, the device is in single
supply operation; this device is powered from AVDD; and the internal voltage regulator is enabled."

So by connecting VREN to AVDD, and connecting DVDD as shown on page, one could theoretically use an (well, three) output pin, thru a resister and a 3.3V zener diode, to power the device. Use the pin as Slave Selectt kind of feature - write it high, perhaps add in delayMicros(200), then read the device, then write it low. I think the space taken up by the resistors and zeners would be better used by a DIP tho.

Pretty much what I had in mind, Bob. Your extra detail was what was required to not blow the chip up ;).

Is the 74LS138 bidirectional? From what I can read it looks like it would only multiplex in one direction which wouldn't work with I2C (I am prepared to be corrected on this).

... so that only the selected device sees the clock line toggling ...

Well that might work of course. The devices see the data line changing but without the clock changing just ignore it.

Ah, isn't there going to be a potential problem with I2C? Unless the Uno is running on 3.3V logic (which it doesn't) then the I2C lines are going to be referencing 5V, but the HMC5843 is only powered with a maximum of 3.3V. The I2C lines (SDA/SCL) - on that device - might not work at those voltage levels, in which case you might need an I2C repeater or buffer. Not sure, just wondering.

Be prepared to be corrected - the Clock only goes one direction!

The spec is not very informative on the I2C levels. Only saying it has internals pullups to DCDD.

I could see a part like this being handy here, and inexpensive too.

http://www.nkcelectronics.com/logic-level-converter.html

CrossRoads:
Be prepared to be corrected - the Clock only goes one direction!

Yes, I agree, which is why I thought that just switching the clock would be OK.

I was about to link this part:

But it looks identical to the one you linked, and yours is somewhat cheaper! The converter has two bidirectional ports and two unidirectional ones. You could use the bidirectional for SDA and the unidirectional for SCL. So you would only need two of them (for the four devices).* Although you still need to switch the AVDD/DVDD.

  • Or do you? You might need only one, because you could connect all four devices to the "low" side of it.

CrossRoads:
I could see a part like this being handy here, and inexpensive too.

http://www.nkcelectronics.com/logic-level-converter.html

Sigh. Only $1 each. I was about to order 20 of them for future use, until I saw the $60 shipping bill.

$60! Whooaaa!

Check back with them - USPS has $14 flat rate postage to any country (Canada/Mexico less). See If they'll ship that way.

That was for USPS.

http://ircalc.usps.gov/?country=10013

If you click on "Priority Mail International Large Flat Rate Box" it quotes "Post Office Price $58.50".

But if I go back and choose "Priority Mail® International Small Flat Rate Box" it quotes $13.95, which is more reasonable.

I'll try to negotiate with NKC Electronics. They don't offer the smaller package on their automated system.

Yeah, press them on it.
I asked UPS about a shipment to Belgium - they wanted $83!
$14 a lot more reasonable.

This should solve your problem.... DssCircuits.com is for sale | HugeDomains

It multiplexes the I2C signal and takes care of the logic level conversion at the same time.

Yes, that will help out the OP.

PCA9544AD SO20 plastic small outline package; 20 leads; body width 7.5 mm SOT163-1 << this one would be workable for hobbyists.
PCA9544APW TSSOP20 plastic thin shrink small outline package; 20 leads; body width 4.4 mm SOT360-1 << 0.65mm spacing is hard
PCA9544ABS HVQFN20 plastic thermal enhanced very thin quad flat package; no leads; 20 terminals; body 5 x 5 x 0.85 mm SOT662-1 << 0.65mm spacing is hard

No easy to use DIP package tho. Too bad, drives up the cost. 74LS138 looks pretty good by comparison.

The problem for Nick is that he is in Australia - and shipping costs there are out of control.
Overseas anywhere seems expensive - UPS wanted $83 to send a USPS Small Flat Box to Belgium. USPS Flat Rate to any Country is $14.

That's why I also offer First Class International shipping with USPS and it's only $2.72 USD.

What would it cost to get FT232RL with the 6 programming pins & 2 LEDs & USB-B Mini on a card?
I like the chip, think I'd prefer a module I could glue down then trying to solder that chip for projects. Want it small, will 'tack' on 6 wires to go to pads on a board, so 0.1" pitch pins not needed.
Have been browsing e-bay, I see modules from HK/China for $5-7, but 2-3 weeks for shipping, and not necessarily trace/component free on the bottom.

How about the Foca (FT232RL Tiny Breakout) from Iteadstudio? Costs $8.90 assembled, but you can get the board for $1.50.

Might be a bit large for what you want. Has some traces on the bottom but no components. If you soldered it yourself you wouldn't need the through-board stuff like the connecting pins.

wayneft:
That's why I also offer First Class International shipping with USPS and it's only $2.72 USD.

Oh is that you? Just ordered some stuff from you. :slight_smile:

No, that's too big.
like this
http://cgi.ebay.com/ft232rl-breakout-/260831532714?pt=LH_DefaultDomain_0&hash=item3cbac556aa
but smaller (no pins down the sides, maybe 2x3 header to connect wires to, or just the holes so its flat on the bottom.
Or leave off the components, just add a mini connector with
+5
+3.3
Gnd
Reset
D+
D-
Rx
Tx
DTR
TxLED
RxLED
Gnd

Maybe space at the end for a 2x6, 2mm connecter like this
http://www.nkcelectronics.com/2mm-2x12-pins-female-so2212.html
or a 1x6 at each end.
The kind of connector this thing could plug onto.

I suppose you thought of this already, but the dead-bug approach?

28 pin SSOP Adapter, but it isn't much smaller than the other board I suggested:

http://www.futurlec.com.au/SMD_Adapters.jsp

I am not delivering finished products with dead bugs. 8) I'm after a reliable deliverable solution that doesn't require a stencil costing 3 times what a batch of boards cost and doesn't require rework of the FT232RL chip.

Like the 28pin SSOP, but overall smaller because fewer connections are needed.