Reading serial port with initial data results in lots of zeros - c

Question

I'm testing a serial implementation and when I open a serial port (I have an Arduino spitting out lines of compass data.) sometimes I get a bunch of zeros initially. I had thought this was leftover data from before, but it does not seem to be (flushing IO doesn't seem to help)
This is a programming language serial implementation written in C and I'm testing this on Linux, but had similar results with Windows.
The strace output shows this on the first read:
read(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 34815) = 1544
write(1, "== ", 3== ) = 3
write(1, "#{\n00000000000000000000000000000"..., 503#{
0000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000583A20353333202020593A20313020
20205A3A2033313920202058733A2033393520202059733A203136312020205A
733A20323933202020483A20302E3436202020413A...) = 503
I have tried adding the following line to clear this data just before the port is closed, after it is opened, and after the attributes it has set:
tcflush(ttyfd, TCIOFLUSH);
but it doesn't seem to help with this issue. Any thoughts about how to clean this up?
The code is from a large project and I have collected some of the relevant parts below, the variables are declared even if not shown but should be clear enough
Opening the port:
ttyfd = open(&devpath[0], O_RDWR | O_NOCTTY | O_NONBLOCK); // ttyUSB0 in this case
Changing settings:
if (speeds[n] == 0) speed = B115200; // invalid, use default
cfsetospeed (&attr, speed);
cfsetispeed (&attr, speed);
// C-flags - control modes:
attr.c_cflag |= CREAD | CS8 | CLOCAL;
// L-flags - local modes:
attr.c_lflag = 0; // raw, not ICANON
// I-flags - input modes:
attr.c_iflag |= IGNPAR;
// O-flags - output modes:
attr.c_oflag = 0;
// Control characters:
// device is non-blocking (polled for changes):
attr.c_cc[VMIN] = 0;
attr.c_cc[VTIME] = 0;
// Make sure OS queues are empty:
tcflush(ttyfd, TCIOFLUSH);
// Set new attributes:
if (tcsetattr(ttyfd, TCSANOW, &attr)) return 2;
This is the Arduino code sending data from an OSEPP Compass module
// OSEPP Compass Sensor Example Sketch
// by OSEPP <http://www.osepp.com>
// Modifications by Chris W. to accommodate declination, scaling and origin adjustment 2013-02-13
// This sketch demonstrates interactions with the Compass Sensor
#include <Wire.h>
const uint8_t sensorAddr = 0x1E; // Sensor address (non-configurable)
const float xOffset = 103.0; // Offset required to adjust x coordinate to zero origin
const float yOffset = -165.0; // Offset required to adjust y coordinate to zero origin
const float declination = 70.1; // Enter magnetic declination mrads here (local to your geo area)
// One-time setup
void setup()
{
// Start the serial port for output
Serial.begin(115200);
// Join the I2C bus as master
Wire.begin();
// Configure the compass to default values (see datasheet for details)
WriteByte(sensorAddr, 0x0, 0x70);
WriteByte(sensorAddr, 0x1, 0x20); // +1.3Ga
// Set compass to continuous-measurement mode (default is single shot)
WriteByte(sensorAddr, 0x2, 0x0);
}
// Main program loop
void loop()
{
uint8_t x_msb; // X-axis most significant byte
uint8_t x_lsb; // X-axis least significant byte
uint8_t y_msb; // Y-axis most significant byte
uint8_t y_lsb; // Y-axis least significant byte
uint8_t z_msb; // Z-axis most significant byte
uint8_t z_lsb; // Z-axis least significant byte
int x;
int y;
int z;
// Get the value from the sensor
if ((ReadByte(sensorAddr, 0x3, &x_msb) == 0) &&
(ReadByte(sensorAddr, 0x4, &x_lsb) == 0) &&
(ReadByte(sensorAddr, 0x5, &z_msb) == 0) &&
(ReadByte(sensorAddr, 0x6, &z_lsb) == 0) &&
(ReadByte(sensorAddr, 0x7, &y_msb) == 0) &&
(ReadByte(sensorAddr, 0x8, &y_lsb) == 0))
{
x = x_msb << 8 | x_lsb;
y = y_msb << 8 | y_lsb;
z = z_msb << 8 | z_lsb;
int xs;
int ys;
int zs;
float gScale = .92; // Scale factor for +1.3Ga setting
float adjx = x - xOffset;
float adjy = y - yOffset;
xs = adjx * gScale;
ys = adjy * gScale;
zs = z * gScale;
float heading = atan2(ys, xs);
heading += declination / 1000; // Declination for geo area
if (heading < 0);
heading += 2*PI;
if (heading > 2*PI)
heading -= 2*PI;
float angle = heading * 180/M_PI;
Serial.print("X: ");
Serial.print(x);
Serial.print(" Y: ");
Serial.print(y);
Serial.print(" Z: ");
Serial.print(z);
Serial.print(" Xs: ");
Serial.print(xs);
Serial.print(" Ys: ");
Serial.print(ys);
Serial.print(" Zs: ");
Serial.print(zs);
Serial.print(" H: ");
Serial.print(heading);
Serial.print(" A: ");
Serial.println(angle);
}
else
{
Serial.println("Failed to read from sensor");
}
// Run again in 1 s (1000 ms)
delay(500);
}
// Read a byte on the i2c interface
int ReadByte(uint8_t addr, uint8_t reg, uint8_t *data)
{
// Do an i2c write to set the register that we want to read from
Wire.beginTransmission(addr);
Wire.write(reg);
Wire.endTransmission();
// Read a byte from the device
Wire.requestFrom(addr, (uint8_t)1);
if (Wire.available())
{
*data = Wire.read();
}
else
{
// Read nothing back
return -1;
}
return 0;
}
// Write a byte on the i2c interface
void WriteByte(uint8_t addr, uint8_t reg, byte data)
{
// Begin the write sequence
Wire.beginTransmission(addr);
// First byte is to set the register pointer
Wire.write(reg);
// Write the data byte
Wire.write(data);
// End the write sequence; bytes are actually transmitted now
Wire.endTransmission();
}
I'll try switch the order of the delay and the serial writing in the arduino code as it may resolve this issue, but the code will remain ineffective for a similar future scenario.

Answer 1

Had a look through the IC's datasheet.
See page 14: Link
Continuous-Measurement Mode. In continuous-measurement mode, the device continuously performs measurements and places the result in the data register. RDY goes high when new data is placed in all three registers. After a power-on or a write to the mode or configuration register, the first measurement set is available from all three data output registers after a period of 2/f_DO and subsequent measurements are available at a frequency of f_DO, where f_DO is the frequency of data output
According to Arduino docs, the Wire library runs at 100KHz. 2/100KHz == 20uS. Assuming your Arduino runs at 16MHz (1 cycle == 62.5ns), you probably aren't waiting long enough to read the registers.
To make your code more robust, I recommend checking the status register's RDY bit before performing any reads. The data is likely correct, but you have to account for the startup time taken by the IC.
If you do this, and find the same issue still occurring, then at least you'll have further isolated the issue.
Alternatively: you can put the 500ms delay at the bottom of your main loop at the top if you just want a quick hack.