Eero Nurkkala said the following on 02/23/2009 03:37 PM: > On Fri, 2009-02-20 at 21:59 +0100, ext Woodruff, Richard wrote: > >> I received the following comment from a HW Apps person whom has dealt with this at the board level. >> >> <comment start> >> Attached is the I2C spec that I have. As I understand it, the I2C only specify the minimum tLow and tHigh (which is not "balanced"). However what is more important is that the appropriate setup and hold time are followed. >> >> If the equal time still meets the setup/hold time then there should not be any issue from a compliance standpoint. >> <comment end> >> >> Regards, >> Richard W. >> > With out board, the tLow and tHigh values are in line with I2C standard > with the patch: > http://marc.info/?l=linux-omap&m=122770723311340&w=2 > > Would that risk the setup and hold times? (If I remember correctly, the > values (setup, hold) were within the I2C standard even with the patch.) > > I think it's a risk not to meet tLow and tHigh. I'm saying, with open > source values with our board, the tLow was not in standard. If using > TRM minimum values, things get even worse. Why? because it states, for > example, Fast Mode SCLL = 5 and SCLH = 7. This means that the low > period is smaller than the high! Shouldn't it be vice versa? (scope > verified). > just couple of views: (Disclaimer - it might be good if someone could re validate my math :( ).. a) tHigh and tLow could be device dependent. for an I2C bus with multiple devices, we probably need a common denominator. the question is - is there a min and max for the tHigh and tLow? some of the datasheets I have seen seem to have this. b) the drivers/i2c/busses/i2c-omap.c does computation based on dev->speed. the equation is based on: tHigh = ( sclh +6 )*iclk period tLow = ( scll +6 )*iclk period the TRM ([2] table18-12(page 2942)) instead speaks of: tHigh = ( sclh +5 )*iclk period tLow = ( scll +7 )*iclk period current i2c driver attempts to put the same value to scll and sclh reg. The result would be that tHigh != tLow, unless I am missing something. I am not entirely sure about the basis of Eero's equation: scll = (scl+3) * iclk sclh = (scl+9) * iclk Now computing a bit, i2c spec [1] says in table 5 for minimum for FS as tLow as 1.3uS and tHigh as 0.6uS. max is not mentioned :(.. OMAP3430 TRM ([2] table 18-13 (page 2943)) says: psc=9, scl = 5, sch =7 - for fclk = 96Mhz. translating this: iclk =96M/(9+1) = 9.6Mhz = 0.104167uSec. tHigh = ( sclh +5 )*iclk = (7+5)*0.104167 = 1.250004 uSec (spec min is 0.6uSec) tLow = ( scll +7 )*iclk =(5+7)*0.104167 = 1.250004 uSec (spec min is 1.3uSec <== I might be missing something here - but does that look like some sort of violation? Now, from a s/w perspective, If we would like to have it so that we can configure tHigh and tLow based on devices, electrical characteristics on a bus, not just speed of the bus, the current implementation would probably need to change(I think). Regards, Nishanth Menon Ref: [1] http://www.nxp.com/acrobat_download/literature/9398/39340011.pdf [2] http://focus.ti.com/pdfs/wtbu/SWPU114I_PrelimFinalEPDF_06_10_2008.pdf -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
