I have an ADXL355 (EVAL-ADXL355-PMDZ) that I am trying to test against a very expensive industrial grade sensor. I am using I2C and I am able to read the device properties and settings as described in the datasheet.
The issue I'm having is how to read the 3 ZDATA (or XDATA, YDATA) registers as a single value. I have tried two approaches. Here is the first:
double values[3];
Wire.beginTransmission(addr);
Wire.write(0x08); // ACCEL_XAXIS
Wire.endTransmission();
Wire.requestFrom(addr, 9, true); // Read 9, 3 for each axis
byte x1, x2, x3;
for (int i = 0; i < 3; ++i){
x3 = Wire.read();
x2 = Wire.read();
x1 = Wire.read();
unsigned long tempV = 0;
unsigned long value = 0;
value = x3;
value <<= 12;
tempV = x2;
tempV <<= 4;
value |= tempV;
tempV = x1;
tempV >>= 4;
value |= tempV;
values[i] = SCALEFACTOR * value;
}
This will produce values that approach 1g for negative gravity and 3g for positive gravity. Also the unloaded axes will sometimes show offscale high instead of -0.0g. They bounce from 0.0 to 4.0 g's. This tells me I have a sign problem which I'm sure comes from using unsigned long. So I attempted to read it as a 16 bit value and retain the sign.
double values[3];
Wire.beginTransmission(addr);
Wire.write(0x08); // ACCEL_XAXIS
Wire.endTransmission();
Wire.requestFrom(addr, 9, true); // Read 9, 3 for each axis
byte x1, x2, x3;
for (int i = 0; i < 3; ++i){
x3 = Wire.read();
x2 = Wire.read();
x1 = Wire.read();
long tempV = 0;
long value = 0;
value = x3;
value <<= 8;
tempV = x2;
value |= tempV;
values[i] = SCALEFACTOR * value;
}
This produced values are good in terms of sign but they are (as expected) much lower in magnitude than they are supposed to be. I tried to create a 20 bit number like this long value:20; but I received
expected initializer before ':' token
same error for int.
How do I properly read from 3 registers to obtain a correct 20 bit value?
First of all, you really want to use unsigned types when using the left and right shift operators (see this question).
Taking a look to the avr-gcc type layout we learn that long are represented on 4 bytes (i.e. 32 bits) so they are long enough (no pun intended) to "hold" your 20 bits numbers (XDATA, YDATA, and ZDATA). On the other hand, int are represented on 2 bytes (i.e. 16 bits) and thus should not be used in your case.
According to the datasheet you linked page 33, the numbers are formatted as two's complement. Your first example correctly set the last 20 bits of your unsigned, 32 bits long value (in particular the left justification handling — right-shifting x1 by four — already looks correct) but the "new" 12 most significants bits are always set to 0.
To perform sign extension, you need to set the "new" 12 most significant bits to 0 if the number is a positive value, 1 if the number is a negative value (adaptation of your first example):
...
value |= tempV;
if (x3 & 0x80) /* msb is 1 so the number is a negative value */
value |= 0xFFF00000;
From there, what you should observe is about the same behaviour as previously: high positive values instead of small negative ones (but even higher than previously). This is caused by the fact that while your value is correct bitwise speaking, it is still intepreted as unsigned. This can be worked around by forcing the compiler to use value as signed:
values[i] = SCALEFACTOR * (long)value;
And now it should be working.
Note that this answer use the fact that your C/C++ implementation use two's complement to represent negative integers. While very rare in practice, the standard allow other representations (see this question for examples).
Here is one way to make it work. It does use bitshifting on a signed value. Various sources have said that this is a potential bug as it is implementation defined. It worked on my platform.
typedef union {
byte bytes[3];
long value:24;
} accelData;
double values[3];
Wire.beginTransmission(addr);
Wire.write(0x08); // ACCEL_XAXIS
Wire.endTransmission();
Wire.requestFrom(addr, 9, true); // Read 9, 3 for each axis
accelData raw;
for (int i = 0; i < 3; ++i){
raw.bytes[2] = Wire.read();
raw.bytes[1] = Wire.read();
raw.bytes[0] = Wire.read();
long temp = raw.value >> 4;
values[i] = SCALEFACTOR * (double)temp;
}
I prefer the solution presented by Alexandre Perrin.
union
{
struct
{
unsigned : 4;
unsigned long uvalue : 20;
};
struct
{
unsigned : 4;
signed long ivalue : 20;
};
unsigned char rawdata[3];
}raw;
for (int i = 0; i < 3; ++i){
raw.bytes[2] = Wire.read(); //if most significant part is transfered first
raw.bytes[1] = Wire.read();
raw.bytes[0] = Wire.read();
values[i] = SCALEFACTOR * (double)raw.ivalue;
}
I'm trying to get CRC32 hash from QByteArray. The problem is that, everytime I run program, it gives different results if using QByteArray::operator=() but right if I use QByteArray::setRawData(). could anyone explain why I'm getting these strange results? Thanks.
crc32 function:
unsigned int crc32_tab[256] = {
0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f,
0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91, 0x1db71064, 0x6ab020f2,
0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b, 0x35b5a8fa, 0x42b2986c,
0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423,
0xcfba9599, 0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190, 0x01db7106,
0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d,
0x91646c97, 0xe6635c01, 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7,
0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa,
0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81,
0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683, 0xe3630b12, 0x94643b84,
0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5, 0xd6d6a3e8, 0xa1d1937e,
0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55,
0x316e8eef, 0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe, 0xb2bd0b28,
0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f,
0x72076785, 0x05005713, 0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69,
0x616bffd3, 0x166ccf45, 0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc,
0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693,
0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
};
...
unsigned int MyClass::crc32(unsigned int crc, const void *buf, unsigned int size)
{
const unsigned int *p;
p = (const unsigned int *)buf;
crc = crc ^~ 0xFFFFFFFF;
while(size--)
{
crc = this->crc32_tab[(crc ^ *p++) & 0xFF] ^ (crc >> 8);
}
return crc ^~ 0xFFFFFFFF;
}
and string to be calculated.
QByteArray crcval = "abc";
MyClass mclass;
QMessageBox::information(0, 0, QString::number(mclass.crc32(0, crcval.constData(), crcval.size()))); // returns random numbers.
...
QByteArray crcval;
crcval.setRawData("abc", 3);
MyClass mclass;
QMessageBox::information(0, 0, QString::number(mclass.crc32(0, crcval.constData(), crcval.size()))); // OK.
Why?
In first case your string converted to QString. So it's byte representation will be a string in utf-16 with trailing zero (total 8 bytes).
In first case you path 4 bytes - a, b, c, \0
In second - to char[3]
What you mean by "random numbers"?
I have int A, B, C. And A is in range 0-9999, B is 0-99, C is 0-99.
Because the function must return only one double, I think of putting them all into one number. Otherwise I need to call function three times.
But I cannot write an efficient code to do this. This will be called millions times, so it should be quite effective, but no ASM.
I need a function double pack3int_to_double(int A, int B, int C) {}
Couldn't you just store A + 1000B + 100000C?
For example, if you wanted to store A = 1234, B = 6, and C = 89, you'd just store
89061234
CCBAAAA
You can then extract the numbers by casting the double to an int and using standard integer division and modulus tricks to recover the individual values.
Hope this helps!
If A<10,000 and B & C <100, A can be expressed with 14 bits, and B & C with 8 bits. Thus you need 30 bits in total.
You could therefore pack/unpack the integers by shifting it to the right place:
int packed = A + B<<14 + C<<22;
A = packed & 0x3FFF; B = (packed >> 14) & 0xFF; C = (packed >> 22) & 0xFF;
Bit shifting is of course MUCH faster than multiply/divide, and you can cast the int to a double and vice versa.
This is technically not legal C code, so you would use this at your own risk:
typedef union {
double x;
struct {
unsigned a : 14;
unsigned b : 7;
unsigned c : 7;
} y;
} result_t;
The C standard doesn't allow using a union member to write a value and a different one to read it out, but I am not aware of a compiler that does the static analysis to diagnose such a problem (it doesn't mean one won't do so in the future). Also, using certain int values may result in a trap representation for a double. But, if you know your system will not generate any trap representations, you can consider using this.
double pack3int_to_double(int A, int B, int C) {
result_t r;
r.y.a = A;
r.y.b = B;
r.y.c = C;
return r.x;
}
void unpack3int_from_double (double X, int *A, int *B, int *C) {
result_t r = { X };
*A = r.y.a;
*B = r.y.b;
*C = r.y.c;
}
You can use out parameters in function call and retrieve all 3 int variables.
You could return a NaN double with the data stored in the mantissa. That gives you 53 bits to utilize. Should be plenty.
http://en.m.wikipedia.org/wiki/NaN
Inspired by your answers, this is what I come up so far. This should be quite efficient, and only 32 bits are used, so the exponent of the double is not touched.
struct pack_abc {
unsigned short a;
unsigned char b, c;
int safety;
};
double pack3int_to_double(int A, int B, int C) {
struct pack_abc R = {A, B, C, 0}; // or 0 could be replaced with something smater, like NaN?
return *(double*)&R;
}
void main() {
int w = 1234, a = 56, d = 78;
int W, A, D, i;
double p = pack3int_to_double(w, a, d);
// we got the data packed into 'p', now let's unpack it
struct pack_abc *R = (struct pack_abc*) & p;
printf("%i %i %i\n", (int)R->a, (int)R->b, (int)R->c);
}