Messages in this thread | | | Date | Mon, 7 Aug 2006 17:58:33 +0300 | From | Tony Lindgren <> | Subject | Re: [PATCH] OMAP: I2C driver for TI OMAP boards #3 |
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Hi,
* Jean Delvare <khali@linux-fr.org> [060805 11:31]: > > > >> + if (armxor_rate > 16000000) > > >> + psc = (armxor_rate + 8000000) / 12000000; > > >> + else > > >> + psc = 0; > > > > >Can you please explain this formula? > > > > The OMAP core uses 8-bit value to divide the system clock (SCLK) and > > generates its own sampling clock (ICLK), and the core logic is sampled > > at clock rate of the system clock for the module, divided by (prescaler value + 1) > > I should have been more precise, I guess. What surprises me are the > numbers themselves. It's frequent to see forumlae of the form > "a = (b + c/2) / c" to divide with proper rounding, but here you have > 2c/3 instread of c/2. My question was more like: is it intentional, or a > typo? Also, with the code above, psc will never have value 1. The "if" > part will always compute to at least 2, and the "else" part to 0. Is > this OK? >
Hmmm, this sounds like a bug somewhere. TRM for 5912 says the I2C clock must be prescaled to be between 7 - 12 MHz [1]. The XOR input clock is typically 12, 13 or 19.2 MHz. So we should have code that produces:
XOR Mhz Divider Prescaler 12 1 0 13 2 1 19.2 2 1
Then again the original old code produces something different too [2]...
I suspect the original code had some hw workarounds and and later code may have a conversion bug somewhere :)
I suggest we keep the code as is for now since it's known to work on all omaps, and then submit a follow-up patch later once we have verified that that code based on the TRM works on all omaps.
Regards,
Tony
[1] http://focus.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=spru681 [2] http://linux-omap.bkbits.net:8080/main/diffs/drivers/i2c/busses/i2c-omap.c@1.12?nav=index.html|src/|src/drivers|src/drivers/i2c|src/drivers/i2c/busses|hist/drivers/i2c/busses/i2c-omap.c - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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