Re: Question about using I2C_IOCTL_EXEC to read data over I2C

[Paul Goyette <paul%whooppee.com@localhost>]

Dave,

As I recall, our I2C driver does not support block transfers.  It is
necessary to read (or write) one byte at a time.  So you would have
to loop from 0xAA-0xBF.

A long time ago I prepared patches for a small subset of our supported
I2C controllers, but they (the patches) are long gone.

Good luck!


On Tue, 17 Aug 2021, Dave Tyson wrote:

> I am trying to get data from a temperature/pressure sensor connected via i2c
> to a banana pi running NetBSD current. I understand the I2C protocol but I am
> having a bit of difficulty understanding what appears on the wire when the
> I2C_IOCTL_EXEC is called with the various op commands. By trial and error I
> seem to have been able to read the data, but want to check a few things in
> case I am doing it all wrong :-)
>
> The device appears at address 0x77 (it's a BMP085) with i2cscan, the data
> sheet indicates the read address=0xEF/write address=0xEE. I just put 0x77 in
> the address field and assume the read/write bit on the wire is added based on
> the op code (I2C_OP_WRITE, I2C_OP_READ etc).
>
> The device has R/O calibration data in 22 contiguous registers starting at
> 0xAA->0xBF. Linux programs seem to grab the data in one go starting at 0xAA.
> The other registers needed to initiate a sensor data grab are R/W - you write
> a control byte into the 0xF4 register, wait a bit and then read the data from
> another register set.
>
> A naive attempt to read the calibration data using:
>
>    command = 0xAA ;
>    iie.iie_op = I2C_OP_READ ;
>    iie.iie_addr = 0x77 ;
>    iie.iie_cmd = &command ;
>    iie.iie_cmdlen = 1 ;
>    iie.iie_buf = &caldata[0] ;
>    iie.iie_buflen = 22;
>    if ((ioctl(iicfd, I2C_IOCTL_EXEC, &iie)) !=0) {
>        printf("read failed %d\n",errno) ;
>        exit(1) ;
>   }
>
> actually seemed to work OK, but I don't understand why!
>
> I had expected to need a I2C_OP_WRITE first followed by a I2C_OP_READ_STOP.
> The former would send a start bit, the device addr/write bit and the target
> register. The latter would send a (re)start bit, device addr/read bit, pull
> the data back and issue a stop. Maybe because the register I am addressing is
> R/O there is no need for a write and what I am doing is correct... (or do I
> need a I2C_OP_READ_STOP)
>
> Could someone explain what actually gets sent on the wire for the various ops:
>
> I2C_OP_READ
> I2C_OP_READ_WITH_STOP
> I2C_OP_WRITE
> I2C_OP_WRITE_WITH_STOP
> I2C_OP_READ_BLOCK
> I2C_OP_WRITE_BLOCK
>
> and what difference block operations make as man ICC(4) is terse to say the
> least.
>
> Cheers,
> Dave
>
>
> !DSPAM:611be3ce51591168618607!

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