Messages in this thread | | | Date | Fri, 23 Dec 2005 11:38:03 +0300 | From | Vitaly Wool <> | Subject | Re: [PATCH 2.6-git] SPI: add set_clock() to bitbang |
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David Brownell wrote:
>>>How exactly that's done is system-specific. Many controllers >>>just have a register per chipselect, listing stuff like SPI mode, >>>clock divisor, and word size. So switching to that chipselect >>>kicks those in automatically ... devices ignore the clock unless >>>they've been selected. >>> >>> >>Hmm, usually clocks are configured for the bus not device. >> >> > >Not a chance. The clock is activated to talk to a given device; >and there's no requirement that all devices on the bus use the >same clock rate. (If one chipselect gives access to a linked series >of devices, clearly they'll all need to be clocked alike. But >that's not a bus, it's just a compound device ... like a big shift >register.) > >I did my homework when putting that API together, and looked at >quite a few SPI controllers. **Not one** of them forces all >their chipselets to use the same clock rate. > > I admit that thw word 'usually' is incorrect here, but still we have two Philips ARM boards where the SPI clock is configured _only_ on the bus, by setting the bus clock divisor on per-message basis. I was also keeping in mind PXA, so it wasn't just bare words...
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