BMP085 : Change the macro to swap
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From: Christoph Mair on 25 Jun 2010 09:40 On Fri, Jun 25, 2010 at 3:00 PM, Datta, Shubhrajyoti <shubhrajyoti(a)ti.com> wrote: > > >> -----Original Message----- >> From: J.I. Cameron [mailto:jic23(a)hermes.cam.ac.uk] On Behalf Of Jonathan >> Cameron >> Sent: Thursday, June 24, 2010 6:23 PM >> To: Datta, Shubhrajyoti >> Cc: linux-kernel(a)vger.kernel.org; Christoph Mair; Andrew Morton >> Subject: Re: [PATCH] BMP085 : Change the macro to swap >> >> On 06/24/10 13:41, Datta, Shubhrajyoti wrote: >> > >> > Changing the macro to swap the bytes as the reason that the first byte >> is the MSB and the next is LSB. >> Are you sure on this one? > > The datasheet says the E2PROM has the bytes as described in the comment below > * > * BMP085 Reg Addr > * parameter | MSB | LSB > * AC1 | 0xAA | 0xAB > ... > > AA has MSB and AB has lsb so it has to be swapped. My idea is that it does not depend on the endianness of the CPU running the code. So I think that swap should happen unconditionally. The I2C reads a string of bytes and the first byte from AA and next from AB and hence an unconditional swap maybe needed. > > Am I missing something. The data is read bytewise and stored in memory starting with MSB-LSM-MSB-LSB-... If you have a big endian CPU, it expects a 16 Bit word to be stored in exactly this format. A little endian CPU expects the LSB to be stored at the lower memory address: LSB-MSB. Therefore the bytes have to be swapped only for little-endian cpus. Best regards, Christoph -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |