I can't reach 1MSPS with my MSP432, what am I doing wrong?

Hello dear e2e,

I want to run my ADC in repeat single channel mode at 1MSPS. For the clock source I select MODCLK at 24MHz, with it I've had the best results. SYSCLK runs at 48MHz. I measure the speed by toggling a pin every time the ADC Interrupt handler is called. On an oscilloscope I read a frequency of 377kHz. Multiply by 2 and you get a sample rate of 754kHz, which is only 3/4 of the maximum achievable rate.

I tried to keep the code as basic as possible and I don't think I can optimise it any further. Why am I not reaching 1MSPS?

~Andy

Here is my code:

#include "msp.h"
#include <stdint.h>


volatile uint16_t A0result;


int main(void)
{
    WDT_A->CTL = WDT_A_CTL_PW |             // Stop watchdog timer
                 WDT_A_CTL_HOLD;

    // Configure GPIO
    P5->SEL1 |= BIT5;   // Enable A/D channel A0
    P5->SEL0 |= BIT5;

    P5->SEL1 &= ~BIT4; // Pulsing pin config
    P5->SEL0 &= ~BIT4;
    P5->DIR |= BIT4;
    P5->OUT &= ~BIT4;


    // Turn on ADC14
    ADC14->CTL0 = ADC14_CTL0_ON |
    		ADC14_CTL0_SSEL__MODCLK |
            ADC14_CTL0_MSC |
			ADC14_CTL0_SHT0__4 | //Shortest sampling time possible
            ADC14_CTL0_SHP |
			ADC14_CTL0_CONSEQ_2; //Repeat single channel

    ADC14->MCTL[0] = ADC14_MCTLN_INCH_0;    // ref+=AVcc, channel = A0;
    ADC14->CTL0 |= ADC14_CTL0_ENC | ADC14_CTL0_SC;

    SCB->SCR &= ~SCB_SCR_SLEEPONEXIT_Msk;   // Wake up on exit from ISR

    __enable_interrupt();
    NVIC->ISER[0] = 1 << ((ADC14_IRQn) & 31);// Enable ADC interrupt in NVIC module
    ADC14->IER0 = ADC14_IER0_IE0;           // Enable ADC14IFG.0

    while(1)
    {
    	__sleep();
        __no_operation();                   // For debugger
    }
}

// ADC14 interrupt service routine
void ADC14_IRQHandler(void)
{
    if (ADC14->IFGR0 & ADC14_IFGR0_IFG0)
    {
        A0result = ADC14->MEM[0];   // Move A0 results, IFG is cleared
        P5->OUT ^= BIT4;
    }
}