Hello! On 17/06/15 20:57, Felipe Balbi wrote: >>>>>>> duty cycle as possible. The reason for this >>>>>>> > >>> > > is that some devices present an erratic behavior >>>>>>> > >>> > > with certain duty cycles. >>>>>>> > >>> > > >>>>>>> > >>> > > One such example is TPS65218 PMIC which fails >>>>>>> > >>> > > to change voltages when running @ 400kHz and >>>>>>> > >>> > > duty cycle is lower than 34%. >>>>>>> > >>> > > >>>>>>> > >>> > > The idea of the patch is simple: >>>>>>> > >>> > > >>>>>>> > >>> > > calculate desired scl_period from requested scl >>>>>>> > >>> > > and use 50% for tLow and 50% for tHigh. >>>>>>> > >>> > > >>>>>>> > >>> > > tLow is calculated with a DIV_ROUND_UP() to make >>>>>>> > >>> > > sure it's slightly higher than tHigh and to make >>>>>>> > >>> > > sure that we end up within I2C specifications. >>>>>>> > >>> > > >>>>>>> > >>> > > Kudos to Nishanth Menon and Dave Gerlach for helping >>>>>>> > >>> > > debugging the TPS65218 problem found on AM437x SK. >>>>>>> > >>> > > >>>>>>> > >>> > > Signed-off-by: Felipe Balbi <balbi@xxxxxx> >>>>> > >> > >>>>> > >> > NAK. >>>>> > >> > This is a direct violation of PHILIPS I2C-bus Specification v.2.1, >>>>> > >> > section 15. >>>>> > >> > Namely, you will have LOW period of SCL clock shorter than required >>>>> > >> > 1.3uS. >>> > > how is this out of spec ? >>> > > >>> > > http://i.imgur.com/jEDlZT7.png >>> > > >>> > > -Width = 1.4us, frequency 373.1kHz, duty cycle of 47.76% >>> > > >>> > > In any case, I have to send v2 anyway (found a bug which would show up >>> > > on frequencies above 400kHz), so I'll resend this patch. >> > >> > If you really target 50% duty cycle and there will be no >> > rounding/truncation error, you will end up with 1.25uS at 400kHz. I >> > understand why you want to make HIGH phase longer, but 50% is a bad >> > target at 400hHz. Probably more safe value? > We can't generate exactly 400kHz anyway, and we're not getting exactly > 50% duty cycle, it just brings it *closer* to that. I even mention the > reason for the DIV_ROUND_UP() there. Let's just go through the math > using TRM equation: > > @400kHz with 12MHz internal clock: > > tLow = (SCLL + 7) * iclk_period > > 1250 = (SCLL + 7) * 1/12MHz > > SCLL = DIV_ROUND_UP(1250, 83) - 7 > SCLL = (1250 + 82)/83 - 7 > SCLL = 9 > > Now if we do the reverse to find actual tLow: > > tLow = (9 + 7) * 83 > tLow = 1328ns > > Likewise if we do it for tHigh: > > tHigh = (SCLH + 5) iclk_period > > SCLH = 1250 / 83 - 5 > SCLH = 10 > > tHigh = 15 * 83 > tHigh = 1245 > > tHigh + tLow (SCL period) = 2573ns. That gives us SCL of 388.65kHz. An The bus rate is actually 12MHz/(SCLH+5+SCLL+7), so it's ~387096Hz. Anyway, it's still possible to have maximum bus rate if SCLH+SCLL=12000000/400000-5-7 Let say SCLH=9, SCLL=9, still within the spec and exactly 400kHz But you need another equations and it's not about 50% duty cycle > error of mere 3% of what we really wanted to achieve. I'd say this is > pretty darn good. > > Now you tell me, how is this *ever* going to be out of spec ? > > DIV_ROUND_UP() and the truncation at internal clock period calculation > makes sure that tLow will be within spec. Instead of blindly NAKing the > patch, you could've gone through this exercise yourself. -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
