SMBus to TI bq20x series Gas gauge chips?

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Has anyone used the CCS I2C functions to talk to a Texas Instruments bq20z75 chip or anything similar? The TI chip acknowledges its address, but all data reads give 0xff. I did modify the I2C_start function as the CCS one is too fast for the SMBus spec and the TI chip would not even ACK its own address.
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Ttelmah · Guest

Turn down the speed of your bus, in the I2C setup.
The timing of the start, is dependant on this setting, and I2C/SMBUS, normally interchange fine, provided:
1) You limit the max operating speed to less than 100K (use for example, 'slow=80000' in the I2C setup).
2) You run with much larger pull-up resistors. Typically something like 20K for SMBUS.

Best Wishes

Guest · Posted: Mon Mar 10, 2008 2:05 am

This may be of interest

ccsinfo.com/forum/viewtopic.php?t=26006&highlight=

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Guest · Posted: Tue Mar 11, 2008 2:50 am

oops , the http part was missing from the post, if you search for

PIC18F2585 Soft I2C Pin selection Problem

you will see code that talks to a gas gauge for a battery,
perhaps it may help

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

This still has me baffled! If I use about slow=45000 in the #use i2c statement I get about the same timing as the Texas Instruments EV2300 demo box, but still I read only 0xff.

I write to address 0x16 and read from 0x17 and the gas gauge chip acknowledges correctly. If I use a different address it does not acknowledge, so I know it sees and recognizes its own address. I dropped the pull-up resistors to 2k so the data and clock edges are real clean. I even added a gratuitous 150 us between bytes because the demo box does that. I have given up just trying to meet the SMB spec. I am trying to meet every idiosyncrasy of the demo box, and still the demo box can talk to the bq20z75 chip and I can't.
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ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

How about posting a short example program showing your problem? Wink

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Finally it is starting to work! I saw that with #use I2C set to "slow" the Start condition was running too fast, so I wrote my own. Then I found out from Ttelmah that you can specify a particular speed for the bus (not mentioned in my manual or version 3.242 help files) and started to slow down the I2C further. Finally I realized my Start function did not do re-start timing to spec. So I went back to the CCS I2C_Start function with slow=40000 and things started to work. Now with fast pull-ups (2k) I can get it to run as fast as slow=60000. Next I will go back to regular 10k SMBus pull-ups and see if I can maintain that speed. My SMBus length is only about 1" plus a 13pF scope probe.

This was supposed to be the easy part!
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The search for better is endless. Instead simply find very good and get the job done.