In my project I am using PIC32MX250F128B controller and 23LC1024 SRAM. I have written a code to sample the analog signal from the vibration sensor. For this I am triggering my ADC with the help of timer 3 and have set the sampling freq at 20khz. First I checked my system by giving 50Hz sine wave to a vibration sensor and took the analog samples from there. At that time I was getting the data as my set sampling frequency (20 khz). But when I checked my system to collect vibration data from the motor(with same sensor) my sampling frequency has reduced to only about 200 samples per second. My code is such as I am sampling one data and then displaying that data through serial monitor then again I am sampling data and displaying it. I have attached my code here. Can someone please help me why I am losing this much sampling frequency? Thank you.

#include <p32xxxx.h>                // include chip specific header file
#include <plib.h>                   // include peripheral library functions
#include <stdlib.h>

// Configuration Bits
#pragma config FNOSC = FRCPLL       // Internal Fast RC oscillator (8 MHz) w/ PLL
#pragma config FPLLIDIV = DIV_2     // Divide FRC before PLL (now 4 MHz)
#pragma config FPLLMUL = MUL_20     // PLL Multiply (now 80 MHz)
#pragma config FPLLODIV = DIV_2     // Divide After PLL (now 20 MHz)


#pragma config FSOSCEN = OFF            // Secondary Oscillator Enable (Disabled)
#pragma config IESO = OFF               // Internal/External Switch Over (Disabled)
#pragma config POSCMOD = OFF            // Primary Oscillator Configuration (Primary osc disabled)
#pragma config OSCIOFNC = ON          // CLKO Output Signal Active on the OSCO Pin (Disabled)
#pragma config FPBDIV = DIV_2           // Peripheral Clock Divisor (Pb_Clk is Sys_Clk/1)
#pragma config FCKSM = CSECME           // Clock Switching and Monitor Selection (Clock Switch Enable, FSCM Enabled)
#pragma config WDTPS = PS1              // Watchdog Timer Postscaler (1:1)
#pragma config WINDIS = OFF             // Watchdog Timer Window Enable (Watchdog Timer is in Non-Window Mode)
#pragma config FWDTEN = OFF             // Watchdog Timer Enable (WDT Disabled (SWDTEN Bit Controls))
#pragma config FWDTWINSZ = WINSZ_75     // Watchdog Timer Window Size (Window Size is 75%)

#pragma config JTAGEN = OFF             // JTAG Enable (JTAG Disabled)
#pragma config ICESEL = ICS_PGx1        // ICE/ICD Comm Channel Select (Communicate on PGEC1/PGED1)
#pragma config PWP = OFF                // Program Flash Write Protect (Disable)
#pragma config BWP = OFF                // Boot Flash Write Protect bit (Protection Disabled)
#pragma config CP = OFF                 // Code Protect (Protection Disabled)
// Defines
#define SYSCLK 40000000L
#define RAM_WRITE_CMD (0x2000000) // top 8 bits -- 24 bits for address this is starting point(2^25))
#define RAM_READ_CMD  (0x3000000) // top 8 bits -- 24 bits for address(this is recoverable exception)

// Macros
// Equation to set baud rate from UART reference manual equation 21-1
#define Baud2BRG(desired_baud)      ( (SYSCLK / (16*desired_baud))-1)

// Function Prototypes
int SerialTransmit(const char *buffer);
unsigned int SerialReceive(char *buffer); //, unsigned int max_size);
int UART2Configure( int baud);

char a2dvals[30000];
int adcptr,num_channels,k,i;
char sampling;
int ADC_RSLT0,totaldata,totaldata1,chunks_sent,data_count,l;
short temp;
BOOL a2don;

volatile unsigned int channel4;
volatile SpiChannel spiChn = SPI_CHANNEL2 ; // the SPI channel to use
volatile int spiClkDiv = 2 ; // 20 MHz max speed for this RAM

int dummy,dummy1,junk;
unsigned char tempstr[5];

inline void Mode16(void){  // configure SPI2 for 16-bit mode
    SPI2CONSET = 0x400;
    SPI2CONCLR = 0x800;

inline void Mode8(void){  // configure SPI2 for 8-bit mode
    SPI2CONCLR = 0x400;
    SPI2CONCLR = 0x800;

inline void Mode32(void){  // configure SPI2 for 32-bit mode
    SPI2CONCLR = 0x400;
    SPI2CONSET = 0x800;
    void modesetbyte(){
    int junk;
    while (SPI2STATbits.SPIBUSY); // wait for it to end of transaction
    junk = ReadSPI2();

    //void modesetsequential(){
    //int junk;
    //while (SPI2STATbits.SPIBUSY); // wait for it to end of transaction
    //junk = ReadSPI2();


void ram_write_byte(int addr, char data){
    int junk;
    // set 32-bit transfer for read/write command ORed with
    WriteSPI2(RAM_WRITE_CMD | addr); // addr not greater than 17 bits
    while (SPI2STATbits.SPIBUSY); // wait for it to end of transaction
    junk = ReadSPI2(); // must always read, even if nothing useful is returned
    Mode8();// actual address// set 8-bit transfer for each byte
    WriteSPI2(data); // data write
    while (SPI2STATbits.SPIBUSY); // wait for it to end of transaction
    junk = ReadSPI2();
    return ;

int ram_read_byte(int addr){
    int junk, data;
    WriteSPI2(RAM_READ_CMD | addr); // addr not greater than 17 bits
    while (SPI2STATbits.SPIBUSY); // wait for it to end of transaction
    junk = ReadSPI2();
    WriteSPI2(junk); // force the read
    while (SPI2STATbits.SPIBUSY); // wait for it to end of transaction
    data = ReadSPI2();
    return data;

void __ISR(_ADC_VECTOR, IPL2AUTO) ADCHandler(void) // Fonction d'interruption Timer 3

        int junk;


        //junk = 0xcccc;

        PORTBbits.RB7 ^= 1;

        temp = ReadADC10(0);

        while (SPI2STATbits.SPIBUSY); // wait for it to end of transaction
        junk = ReadSPI2();

        while (SPI2STATbits.SPIBUSY); // wait for it to end of transaction
        junk = ReadSPI2();

        //a2dvals[k] = (temp);
        if (k>totaldata1)// && sampling == 's')
            T3CONCLR = 0x8000;
            chunks_sent = 0;
            totaldata = k/2;
            k = 1;
            PORTBbits.RB7 = 0;


void main(void)
    char   buf[1024];       // declare receive buffer with max size 1024

    //volatile SpiChannel spiChn = SPI_CHANNEL2 ;   // the SPI channel to use
    //volatile int spiClkDiv = 4 ; // 20 MHz max speed for this RAM

    PPSOutput(2, RPB5, SDO2);//SDO2 (MOSI) is in PPS output group 2, could be connected to RPB5 which is pin 14
    PPSInput(3,SDI2,RPA2);//SDI2 (MISO) is PPS output group 3, could be connected to RPA2 which is pin 9


    SpiChnOpen(spiChn, SPI_OPEN_ON | SPI_OPEN_MODE16 | SPI_OPEN_MSTEN | SPI_OPEN_CKE_REV , spiClkDiv);

    PPSInput (2, U2RX, RPB11); //Assign U2RX to pin RPB11 -- Physical pin 22 on 28 PDIP
    PPSOutput(4, RPB10, U2TX); //Assign U2TX to pin RPB10 -- Physical pin 21 on 28 PDIP


    UART2Configure(9600);  // Configure UART2 for a baud rate of 9600
    U2MODESET = 0x8000;     // enable UART2

    ANSELBbits.ANSB2 = 1;   // set RB2 (AN4) to analog
    TRISBbits.TRISB2 = 1;   // set RB2 as an input

    //adcConfigureManual();   // Configure ADC
    //AD1CON1SET = 0x8000;    // Enable ADC


        // the ADC ///////////////////////////////////////
        // configure and enable the ADC
            CloseADC10();   // ensure the ADC is off before setting the configuration

    // define setup parameters for OpenADC10
    // Turn module on | ouput in integer | trigger mode auto | enable autosample
        // ADC_CLK_AUTO -- Internal counter ends sampling and starts conversion (Auto convert)
        // ADC_AUTO_SAMPLING_ON -- Sampling begins immediately after last conversion completes; SAMP bit is automatically set
        // ADC_AUTO_SAMPLING_OFF -- Sampling begins with AcquireADC10();

    // define setup parameters for OpenADC10
    // ADC ref external  | disable offset test | disable scan mode | do 1 sample | use single buf | alternate mode off
    // Define setup parameters for OpenADC10
        // use peripherial bus clock | set sample time | set ADC clock divider
        // ADC_CONV_CLK_Tcy2 means divide CLK_PB by 2 (max speed)
        // ADC_SAMPLE_TIME_5 seems to work with a source resistance < 1kohm

    // define setup parameters for OpenADC10
    // set AN4 and  as analog inputs
    #define PARAM4  ENABLE_AN4_ANA 

    // define setup parameters for OpenADC10
    // do not assign channels to scan
    #define PARAM5  SKIP_SCAN_ALL

    // use ground as neg ref for A | use AN4 for input A     
    // configure to sample AN4 
    SetChanADC10( ADC_CH0_NEG_SAMPLEA_NVREF | ADC_CH0_POS_SAMPLEA_AN4 ); // configure to sample AN4 
    OpenADC10( PARAM1, PARAM2, PARAM3, PARAM4, PARAM5 ); // configure ADC using the parameters defined above
     ConfigIntADC10(ADC_INT_PRI_2 | ADC_INT_ON);

        EnableADC10(); // Enable the ADC

        OpenTimer3(T3_ON | T3_SOURCE_INT | T3_PS_1_1,2000);


        mPORTBSetPinsDigitalOut(BIT_7);    //Set port as output
        PORTBbits.RB7 = 0;

    SerialTransmit("Hello! Enter 'a' to do ADC conversion \r\n");

    unsigned int rx_size;

    while( 1){
        rx_size = SerialReceive(buf); //, 1024);     // wait here until data is received
        SerialTransmit(buf);                   // Send out data exactly as received





   return 1;
} // END main()

/* UART2Configure() sets up the UART2 for the most standard and minimal operation
 *  Enable TX and RX lines, 8 data bits, no parity, 1 stop bit, idle when HIGH
 * Input: Desired Baud Rate
 * Output: Actual Baud Rate from baud control register U2BRG after assignment*/
int UART2Configure( int desired_baud){

    U2MODE = 0;         // disable autobaud, TX and RX enabled only, 8N1, idle=HIGH
    U2STA = 0x1400;     // enable TX and RX
    U2BRG = Baud2BRG(desired_baud); // U2BRG = (FPb / (16*baud)) - 1

    // Calculate actual assigned baud rate
    int actual_baud = SYSCLK / (16 * (U2BRG+1));

    return actual_baud;
} // END UART2Configure()

/* SerialTransmit() transmits a string to the UART2 TX pin MSB first
 * Inputs: *buffer = string to transmit */
int SerialTransmit(const char *buffer)
    unsigned int size = strlen(buffer);
        while( U2STAbits.UTXBF);    // wait while TX buffer full
        U2TXREG = *buffer; // send single character to transmit buffer

        buffer++;                   // transmit next character on following loop
        size--;                     // loop until all characters sent (when size = 0)


    while( !U2STAbits.TRMT);        // wait for last transmission to finish

    return 0;

/* SerialReceive() is a blocking function that waits for data on
 *  the UART2 RX buffer and then stores all incoming data into *buffer
 * Note that when a carriage return '\r' is received, a nul character
 *  is appended signifying the strings end
 * Inputs:  *buffer = Character array/pointer to store received data into
 *          max_size = number of bytes allocated to this pointer
 * Outputs: Number of characters received */
unsigned int SerialReceive(char *buffer) //, unsigned int max_size)
    //unsigned int num_char = 0;

    /* Wait for and store incoming data until either a carriage return is received
     *   or the number of received characters (num_chars) exceeds max_size */
        while( !U2STAbits.URXDA);   // wait until data available in RX buffer
        *buffer = U2RXREG;  // empty contents of RX buffer into *buffer pointer

        if (*buffer == 'a')
             num_channels = 1;
             totaldata1 = 25500;
             T3CONSET = 0x8000;



            //for (i=0;i<5000;i++);
            int temp1=ram_read_byte(0x0) ;


            WriteSPI2(junk); // force the read
             while (SPI2STATbits.SPIBUSY); // wait for it to end of transaction
             temp1 = (ReadSPI2()&0x03)<<8;
            WriteSPI2(junk); // force the read
             while (SPI2STATbits.SPIBUSY); // wait for it to end of transaction
             temp1 += ReadSPI2();
            dummy = temp1/1000 ;
            tempstr[0] = dummy + 0x30;
            dummy1 = temp1- dummy*1000;
            dummy = dummy1/100;
            tempstr[1] = dummy + 0x30;
            dummy1 = dummy1 - dummy*100;
            dummy = dummy1/10;
            tempstr[2] = dummy + 0x30;
            dummy1 = dummy1 - dummy*10;
            tempstr[3] = dummy1 + 0x30;
            //tempstr[4] = "\0";

            printf("%c%c%c%c \n", tempstr[0],tempstr[1],tempstr[2],tempstr[3]);





    return 1;
}// END SerialReceive()
  • 1
    \$\begingroup\$ At 9600 baud, you can send, maximally, 600 2-byte samples per second across a serial bus \$\endgroup\$ Nov 12, 2018 at 16:41
  • \$\begingroup\$ @ScottSeidman Thank you for your response but is there any way I can send 15k samples per second across the serial bus? \$\endgroup\$ Nov 12, 2018 at 16:51
  • \$\begingroup\$ If you want to send samples faster, you need to increase the baud rate. Asynchronous serial takes 10 bits/character (8 data bits, one start, one stop). Figure out how many characters you need to send every second (including spaces, commas, linefeeds, etc.), multiply by ten, and you have the lowest baud rate that'll work. OR think of where you can stash that data so that a computer can read it out later. \$\endgroup\$
    – TimWescott
    Nov 12, 2018 at 17:34

1 Answer 1


You will not be able to sustain that data rate through standard serial communication and OS drivers. The maximum normal data rate is 115200bps, which translates to 11520 bytes per second (8-N-1), or ~5700 samples per second at 2 bytes per sample (with no intervening commas, carriage returns, or tabs). Some UARTs and programs might be able to handle faster data rates than that, but you are outside of standard territory.

Even if you use a USB-capable micro controller, the OS driver in your computer would limit your data rate. Although in principle 2Mb/s is possible with some Arduino boards, your PC driver will determine what is the maximum data rate you will achieve. While a Mac or Linux serial driver would be able to handle such data rates, a standard Windows USB serial driver would quickly exceed its limits.

If you do not need continuous sampling and you have enough internal memory, store your samples in memory first and then, after you are done, send the data to the PC. That way the communications will not interfere with your sampling rate. It is a good idea to do this anyway, provide some local buffering so that your sampling rate is not affected by small hiccups in communications.


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