大兵萊恩一路直前: 做實驗, 寫入 EEPROM 的速度能否跟得上 MPU6050 的數據產出? (Arduino, MPU-6050, EEPROM)

為了測速球計畫, 萊恩大兵得找個方法讓 Arduino 能將 MPU6050 測得的數據暫存起來. Arduno Uno 只有 1KB 的 EEPROM 空間, 萊恩大兵本來是不考慮的. 後來想說做個實驗也好, 換個不同思考角度, 也許 1KB 夠用也說不定.

本來以為這實驗小蛋糕一片, 結果.. 很可怕. 寫入和讀出兩邊的數值比較起來, 都是前數十筆資料不對, 而後面百多筆資料是正確的.

第17筆資料之後, 才兩邊一致.

第33筆資料之後, 才兩邊一致.

不懂這是怎麼回事.. 難道寫入 EEPROM 之前要先 warm up 嗎? 萊恩大兵需要有經驗的朋友指點一下. 謝謝.

(2014/8/25 更新-1) 感謝各位路過朋友的指點, 這題目在臉書的討論串1和討論串2.
(2014/8/25 更新-2) 萊恩大兵改寫程式, 解決了寫入與讀取不一致的問題.

宣告一個 array 來暫存 MPU6050 ax/ay/az 的輸出值.
找個適當時機做一次寫入.
寫入 EEPROM 時, 還要 delay 夠長一段時間, 才能去中斷程式執行. 如果等不夠久的話, 這次反而會造成讀出與寫入數值相比, 變成是前半段一致, 後半段不一致的情況.

新版本程式碼如下:

// Based on I2Cdevlib, MPU6050_raw
// Experiments to verify if any data loss when writing MPU6050 outputs to EEPROM 

#include "I2Cdev.h"
#include "MPU6050.h"
#include <EEPROM.h>
#include "eeprom_anything.h"

// Arduino Wire library is required if I2Cdev I2CDEV_ARDUINO_WIRE implementation
// is used in I2Cdev.h
#if I2CDEV_IMPLEMENTATION == I2CDEV_ARDUINO_WIRE
    #include "Wire.h"
#endif

MPU6050 accelgyro;
int16_t ax, ay, az;
int16_t gx, gy, gz;

// [eeprom] 
struct accelData{
  int16_t ax;
  int16_t ay;
  int16_t az;
};
accelData acc;
int accIdx = 0;
int accMem = 0;
int writeOnce = 0;

#define NUM 170
//int max = 1024 / sizeof(acc);
accelData accArray[NUM];

void setup() {
    // join I2C bus (I2Cdev library doesn't do this automatically)
    #if I2CDEV_IMPLEMENTATION == I2CDEV_ARDUINO_WIRE
        Wire.begin();
    #elif I2CDEV_IMPLEMENTATION == I2CDEV_BUILTIN_FASTWIRE
        Fastwire::setup(400, true);
    #endif

    // initialize serial communication
    // (38400 chosen because it works as well at 8MHz as it does at 16MHz, but
    // it's really up to you depending on your project)
    Serial.begin(38400);

    // initialize device
    Serial.println("Initializing I2C devices...");
    accelgyro.initialize();

    // verify connection
    Serial.println("Testing device connections...");
    Serial.println(accelgyro.testConnection() ? "MPU6050 connection successful" : "MPU6050 connection failed");
}

void loop() {
    /*int n = writeEEPROM();
    if (n >= NUM 200 && !writeOnce)
    {
        EEPROM_writeAnything(0, accArray);
        writeOnce = 1;
        delay(5000); // give enough time to write EEPROM
        Serial.println("Now you can stop this program...");
    }
    */
        
    dumpEEPROM();
}

int writeEEPROM()
{
    // read raw accel/gyro measurements from device
    accelgyro.getMotion6(&ax, &ay, &az, &gx, &gy, &gz);

    // waive the first 200 sensor data
    if (accIdx >= 200 && accIdx < NUM+200)
    {
        int n = accIdx - 200;
        Serial.print("Idx: "); Serial.print(n); 
        Serial.print(", ax: "); Serial.print(ax); accArray[n].ax = ax;
        Serial.print(", ay: "); Serial.print(ay); accArray[n].ay = ay;
        Serial.print(", az: "); Serial.println(az); accArray[n].az = az;
        //accMem  = EEPROM_writeAnything(accMem, acc);
    }
    return accIdx  ;
}

void dumpEEPROM()
{
  accelData accTmp;
  int tmpMem = 0;
  for (int i=0; i<NUM; i  )
  {
    tmpMem  = EEPROM_readAnything(tmpMem, accTmp);
    Serial.print("Idx: "); Serial.print(i);
    Serial.print(", ax: "); Serial.print(accTmp.ax);
    Serial.print(", ay: "); Serial.print(accTmp.ay);
    Serial.print(", az: "); Serial.println(accTmp.az);    
  }
}

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萊恩大兵這個實驗的規劃如下:

(1) 怎麼讀寫 EEPROM?

這個簡單, 參考 Cooper Maa (可讀寫任何資料的 EEPROM 函式) 或葉難 (Arduino：自訂整組資料讀寫EEPROM) 的文章, 有現成的教學指引. 萊恩大兵直接引用了葉難的 eeprom_anything.h.

(2) 寫入 EEPROM vs MPU6050 數據產出, 速度對決

講起來這是個假議題. 如果寫 EEPROM 跟不上, 那就把 MPU6050 量測輸出調慢一點啊. 是這樣沒錯, 錯的就在萊恩大兵不懂怎麼去調慢 MPU6050 的時脈. XD.
只好拿現成的東西來改. i2cdevlib 的示範程式 MPU6050_raw 讓 MPU6050 的數據輸出間隔為 10ms. 就用這標準來做實驗.

(3) 改寫 MPU6050_raw, 包括:

將 ax, ay, az 的值寫入 EEPROM, 共 170 筆 [1024/(2+2+2)]. 同時也將 ax, ay, az 的值倒在 serial port monitor 上.
再從 EEPROM 去讀出值來, 兩邊做比對.

程式碼如下:

// Based on I2Cdevlib, MPU6050_raw
// Experiments to verify if any data loss when writing MPU6050 outputs to EEPROM 

#include "I2Cdev.h"
#include "MPU6050.h"
#include <EEPROM.h>
#include "eeprom_anything.h"

// Arduino Wire library is required if I2Cdev I2CDEV_ARDUINO_WIRE implementation
// is used in I2Cdev.h
#if I2CDEV_IMPLEMENTATION == I2CDEV_ARDUINO_WIRE
    #include "Wire.h"
#endif

MPU6050 accelgyro;
int16_t ax, ay, az;
int16_t gx, gy, gz;

// [eeprom] 
struct accelData{
  int16_t ax;
  int16_t ay;
  int16_t az;
};
accelData acc;
int accIdx = 0;
int accMem = 0;
int max = 1024 / sizeof(acc);

void setup() {
    // join I2C bus (I2Cdev library doesn't do this automatically)
    #if I2CDEV_IMPLEMENTATION == I2CDEV_ARDUINO_WIRE
        Wire.begin();
    #elif I2CDEV_IMPLEMENTATION == I2CDEV_BUILTIN_FASTWIRE
        Fastwire::setup(400, true);
    #endif

    // initialize serial communication
    // (38400 chosen because it works as well at 8MHz as it does at 16MHz, but
    // it's really up to you depending on your project)
    Serial.begin(38400);

    // initialize device
    Serial.println("Initializing I2C devices...");
    accelgyro.initialize();

    // verify connection
    Serial.println("Testing device connections...");
    Serial.println(accelgyro.testConnection() ? "MPU6050 connection successful" : "MPU6050 connection failed");
}

void loop() {
    //writeEEPROM();
    dumpEEPROM();
}

void writeEEPROM()
{
    // read raw accel/gyro measurements from device
    accelgyro.getMotion6(&ax, &ay, &az, &gx, &gy, &gz);

    // waive the first 100 sensor data
    if (accIdx >= 200 && accIdx < max + 200)
    {
        Serial.print("Idx: "); Serial.print(accIdx-200); 
        Serial.print(", ax: "); Serial.print(ax); acc.ax = ax;
        Serial.print(", ay: "); Serial.print(ay); acc.ay = ay;
        Serial.print(", az: "); Serial.println(az); acc.az = az;
        accMem  = EEPROM_writeAnything(accMem, acc);
    }
    accIdx  ;
}

void dumpEEPROM()
{
  accelData accTmp;
  int tmpMem = 0;
  for (int i=0; i<max; i  )
  {
    tmpMem  = EEPROM_readAnything(tmpMem, accTmp);
    Serial.print("Idx: "); Serial.print(i);
    Serial.print(", ax: "); Serial.print(accTmp.ax);
    Serial.print(", ay: "); Serial.print(accTmp.ay);
    Serial.print(", az: "); Serial.println(accTmp.az);    
  }
}

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未來, 這技術應該可以用在測速球計畫上面.

(1) 寫 EEPROM 的作法, 本質上也是將資料暫存起來, 等以後再找時機將資料倒出來. 這樣來看, 應該可以是測速球的實作選項之一.

(2) 1KB 空間, 可以存多少資料?

根據先前手推車實驗得到的 accel 圖表, 施力的作用時間約為 300ms. (y軸的peak) 而一筆資料佔空間 6bytes (ax, ay, az: int type, 16bit value), 1KB 可記錄 170筆, 170*10ms = 1700ms. 應該是足夠的..