BMP180 allow to read calibration but fail to read data

Question

I have BMP180 connected to my RPi, I can read calibration data from it, but when I try to get pressure / temperature data -- I get 80 00 for temp and 80 00 00 for pressure (mean -- 'just-after-reboot value') the communication log (reading data part) looks like:

device <--   f4
delay for (asked 50us) 0s 0ms 57us
device <--   2e
delay for (asked 50us) 0s 0ms 56us
delay (asked 5 ms) for 0s 5ms 139us
device [f6] ->  80 00
device <--   f4
delay for (asked 50us) 0s 0ms 56us
device <--   b4
delay for (asked 50us) 0s 0ms 393us
delay (asked 14 ms) for 0s 14ms 149us
device [f6] ->  80 00 00
bcm085: t=51.1 p=104462
chipid: 85 version: 2 Calibration data:  ac1=0198 ac2=ffb8 ac3=c7d1 ac4=7fe5 ac5=7ff5 ac6=5a71  b1=182e  b2=0004  mb=8000  mc=ddf9  md=0b34 Raw data:  rt=8000 rp=020000

50 µs delays after each read/write operations can be dropped from sequence with same result. also, increasing wait times from 5 ms to any -- won't help.

Communication code:

#define BMP180_READ_ADDRESS 0x77
#define BMP180_WRITE_ADDRESS 0x77

#define BMP085_REG_CALIBRATION_DATA_ADDR 0xAA
#define BMP085_REG_CHIPID       0xD0
#define BMP085_REG_VERSION      0xD1
#define BMP085_CMD_CONTROL      0xf4
#define BMP085_REG_TEMPDATA     0xf6
#define BMP085_REG_PRESDATA     0xf6
#define BMP085_CMD_READTEMP     0x2e
#define BMP085_CMD_READPRES     0x34
#define BMP085_CMD_SOFTRESET    0xE0

// get raw temperature
bcm2835_i2c_setSlaveAddress(BMP180_WRITE_ADDRESS);
i2c_reg_set8(BMP085_CMD_CONTROL, BMP085_CMD_READTEMP);

rpi_delay(5);

char tempReadBuf[2];
i2c_read_reg(BMP085_REG_TEMPDATA, tempReadBuf, sizeof(tempReadBuf));
tempRawData = (int)tempReadBuf[0] << 8 | (int)tempReadBuf[1];

// get raw pressure
i2c_reg_set8(BMP085_CMD_CONTROL, BMP085_CMD_READPRES + (oversampling << 6));
rpi_delay(2 + (3 << oversampling));


char presReadBuf[3];
i2c_read_reg(BMP085_REG_PRESDATA, presReadBuf, sizeof(presReadBuf));
presRawData = (int)presReadBuf[0] << 16 | (int)presReadBuf[1] << 8 | (int)presReadBuf[2];

presRawData >>= (8-oversampling);

I use libbcm2835 library for access device, without any other libs like wiringPi or smbus. i2c_* and rpi_delay functions are wrappers over libbcm2835 and they do work with other devices.

Can somebody help me please with finding an issue?

Update: more detailed code on pastebin:

• bmp.cpp : http://pastebin.com/NGGWCByp
• i2c_rw.cpp: http://pastebin.com/Ncpg13ZK (warppers over libbcm2835 i2c read/write functions)
• tools.cpp : 9BAAVGEc (wrappers over libbcm2835 delay functions)

Answer 1

the borsch documentation for the chip is available at: http://ae-bst.resource.bosch.com/media/products/dokumente/bmp180/BST-BMP180-DS000-12.pdf

the chip documentation does not agree with the posted code.

Please read/understand the documentation, then design the code.

Amongst other things, the three byte pressure is not sequential bytes and the way to tell if a conversion is complete is to read the control register and look at bit 0x40, which will be 0 when the conversion is complete. The oversampling count is set by bits 0xC0 in the control register. and several other problems with the posted code.

If any more misunderstandings arise about the documentation or how to write the code, then update the question

register 0xD1 is not defined, so no version available.

Please read the documentation.

Answer 2

When I had a similar problem, I found that writing the commands in two separate 1-byte write(2) calls leads to this: I was able to read calibration data, but UT and UP reads resulted in 0x8000 and 0x800000 respectively. After I changed my write(2) of Start Measurement commands from two 1-byte writes to a single 2-byte write, I started to get sensible output.

If this still didn't help, I suggest you to compare the result of running strace over simpletest.py from this GitHub repo with the strace results for your program. This was how I guessed to try 2-byte writes which helped me.