Reading / writing from using I2C on Linux

Question

I'm trying to read/write to a FM24CL64-GTR FRAM chip that is connected over a I2C bus on address 0b 1010 011.

When I'm trying to write 3 bytes (data address 2 bytes, + data one byte), I get a kernel message ([12406.360000] i2c-adapter i2c-0: sendbytes: NAK bailout.), as well as the write returns != 3. See code below:

#include <linux/i2c-dev.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>

int file;
char filename[20];
int addr = 0x53; // 0b1010011; /* The I2C address */
uint16_t dataAddr = 0x1234;
uint8_t val = 0x5c;
uint8_t buf[3];

sprintf(filename,"/dev/i2c-%d",0);
if ((file = open(filename,O_RDWR)) < 0)
    exit(1);

if (ioctl(file,I2C_SLAVE,addr) < 0)
    exit(2);

buf[0] = dataAddr >> 8;
buf[1] = dataAddr & 0xff;
buf[2] = val;

if (write(file, buf, 3) != 3)
    exit(3);

...

However when I write 2 bytes, then write another byte, I get no kernel error, but when trying to read from the FRAM, I always get back 0. Here is the code to read from the FRAM:

uint8_t val;

if ((file = open(filename,O_RDWR)) < 0)
    exit(1);

if (ioctl(file,I2C_SLAVE,addr) < 0)
    exit(2);

if (write(file, &dataAddr, 2) != 2) {
    exit(3);

if (read(file, &val, 1) != 1) {
    exit(3);

None of the functions return an error value, and I have also tried it with:

#include <linux/i2c.h>

struct i2c_rdwr_ioctl_data work_queue;
struct i2c_msg msg[2];
uint8_t ret;

work_queue.nmsgs = 2;
work_queue.msgs = msg;

work_queue.msgs[0].addr = addr;
work_queue.msgs[0].len = 2;
work_queue.msgs[0].flags = 0;
work_queue.msgs[0].buf = &dataAddr;

work_queue.msgs[1].addr = addr;
work_queue.msgs[1].len = 1;
work_queue.msgs[1].flags = I2C_M_RD;
work_queue.msgs[1].buf = &ret;

if (ioctl(file,I2C_RDWR,&work_queue) < 0)
    exit(3);

Which also succeeds, but always returns 0. Does this indicate a hardware issue, or am I doing something wrong?

Are there any FRAM drivers for FM24CL64-GTR over I2C on Linux, and what would the API be? Any link would be helpful.

Answer 1

I do not have experience with that particular device, but in our experience many I2C devices have "quirks" that require a work-around, typically above the driver level.

We use linux (CELinux) and an I2C device driver with Linux as well. But our application code also has a non-trivial I2C module that contains all the work-around intelligence for dealing with all the various devices we have experience with.

Also, when dealing with I2C issues, I often find that I need to re-acquaint myself with the source spec:

http://www.nxp.com/acrobat_download/literature/9398/39340011.pdf

as well as the usage of a decent oscilloscope.

Good luck,

Above link is dead, here are some other links:

http://www.nxp.com/documents/user_manual/UM10204.pdf and of course wikipedia: http://en.wikipedia.org/wiki/I%C2%B2C

Answer 2

The NAK was a big hint: the WriteProtect pin was externally pulled up, and had to be driven to ground, after that a single write of the address followed by data-bytes is successful (first code segment).

For reading the address can be written out first (using write()), and then sequential data can be read starting from that address.