I2C Pull-up Resistors

[jseveret]

Hi Andrew,

Yes, I have access to the customer download area and found the memo you're talking about. The documentation for the StenSat board we're using did not come with a schematic, unfortunately. I'll have to contact Ivan to get it from him. Once I have the schematic, I'll forward it to you.

Thanks for the help!

~Jared

[aek]

OK.

The idea behind the I2C isolator in the memo is that by using the isolator, you "move" the system's I2C lines over to SCL_SYS and SDA_SYS.

The trouble you have is that you have Rev B, but SCL_SYS and SDA_SYS are (only) on Rev C and higher. Assuming the StenSat radio has the isolator and has the newer, 104-pin CSK connectors, then this means that from the StenSat board "and higher" I2C will be on SCL_SYS and SDA_SYS.

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[jseveret]

The StenSat radio we're using does in fact have the updated 104-Pin CSK connector (interfaces perfectly with the 104-Pin CSK socket on our dev board). I just sent a request to Ivan for the schematic.

When you say that...
"StenSat board "and higher" I2C will be on SCL_SYS and SDA_SYS"
...do you mean every module plugged into the CSK bus above the StenSat board will be using SCL_SYS and SDA_SYS?

Also, if SCL_SYS and SDA_SYS do not exist on the Rev B Dev boards, then will it even be possible to get our Dev board properly communicating with the StenSat board?

Thanks.

[aek]

There is no 104-pin CSK connector on Rev B Dev Boards.

Rev B and Test Board B (which is the "fix" for the SD card interface for Rev B) are all 80-pin.

Rev C and later are 104 pin.

Assuming Ivan's radio has the I2C isolator, then all contiguous modules that have 104-pn CSK connectors will have SCL_SYS and SDA_SYS as long as Ivan's radio is plugged into that stack.

IOW, one module carries P3.1 & P3.3 (on the 80- and 104-pin CSK bus) to SCL_SYS and SDA_SYS (only on the 104-pin CSK bus). In your case, it's Ivan's radio that does this.

So,

1) All I2C slaves are interconnected via SCL_SYS and SDA_SYS. The MSP430 Master (P3.1 and P3.3) is connected to SCL_SYS and SDA_SYS via the isolator. If you were to do other I2C Masters, they would connect directly to SCL_SYS and SDA_SYS.

2) Yes. Assuming the isolator on Ivan's board works correctly.

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[This message has been edited by aek (edited March 03, 2008).]

[aek]

Got the schematic ... the isolator is implemented and the pull-up values seem fine.

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[jseveret]

I'm a bit confused about the rev. of our Dev board now. The back of our board says:

705-00194 rev B
(c) 2006

But the board definitely has 104 pins total between the H1 and H2 connectors (I have 2 50-pin pinheaders in there for testing and there are 4 sockets left).

So, does that mean we have a rev B or rev C board?

Also, I just sent the Stensat Board schematic to info@pumpkininc.com.

Thanks.

[aek]

The front of the board says Rev C in the silkscreen ... it's a mistake in the Layer 4 metal.

So you have a Rev C.

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[jseveret]

Hi,

I'm trying to setup my CSK Dev Board (rev B) to communicate with a radio module from the StenSat group via the I2C bus on USART0. The I2C bus requires a pull-up resistor on both SDA (P3.1) and SCL (P3.3) signals.

Looking through the Dev Board schematic, it looks like the only resistors on these lines are part of the SD/MMC interface, but they don't follow the same configuration for Rp and Rs resistors described in the Phillips Semiconductor I2C Bus spec. v2.1 (p.39).

So, here are my questions:

1) Will we need to add pull-up resistors to implement our I2C SDA and SCL lines?

2) If yes, will these resistors interfere with the circuitry already used for the SD/MMC interface?

3) Could you recommend a value for Rp if we need to add these?

Thanks!

~Jared Everett
North Carolina State University
HiPerDSP Lab
(Collaborating with UF-Space Systems Group)

[aek]

Hi Jared.

Rev A - Rev C do not have direct support for I2C, so there are no I2C pullup resistors.

Did UF give you a download key for the CubeSat Customer Area? In it is a Memo on how to use I2C with the CubeSat Kit. You need an isolator circuit that lest the SD/MMC circuitry coexist with the I2C circuitry.

The non-I2C StenSat module was developed before the SD/MMC module. The I2C version is more recent and falls into the "any I2C device for the CubeSat Kit" category. Therefore it needs the isolator circuitry outlined in said memo. I _think_ that Ivan implemented that isolator on the StenSat I2C module, but I'm not sure (I don't have schematics here). Without schematics for the radio I can't be of more help.

Send me the schematics by email and I'll look at them. Use any of the prescribed contact emails.

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[applewood]

Can you tell us what the values of Pull-up resistors on the I2C Clock and Data lines are?

Thanks