Electronic – MSP430 Illegal Instruction

exceptioninterruptsmsp430reset

A few questions here along the same lines, I'm looking at the datasheet for the MSP430x2xx family (I'm using the MSP430F235 FWIW). It says there in section 2.2.4 that "illegal instruction fetch" is an interrupt source for the Reset vector. Then separately under 2.2.1 under the topic of Non-Maskable Interrupts (NMI), "An access violation to the flash memory" is cited as a source.

(1) Can anyone explain to me the difference between (and/or causes of) these two apparently distinct event sources (with different apparently different consequences)?

(2) Can I write an ISR for the NMI source, and make it act like a soft reset by reinitializing the stack pointer and "recursively" calling main or something like that, so that the register and RAM (global variable) state isn't altered?

(3) Assuming I can't intercept the one that results in a Reset vector (which I presume also comes along with a reinitialization of all the registers?), can I at least detect at startup that it happened by checking some status bit to decide what to do about it, and if so which status register/bit would that be?

Best Answer

Illegal instruction means that it read an opcode, but that opcode does not match to any valid machine instruction.

Access violation means that you tried to use a type of memory access that is not supported by the device. For a flash, that usually means you tried to write to it without using the proper procedure for writing or erasing a page.

Access violation can also mean that you tried to execute a privileged instruction while in a user mode, though I'm not sure MSP430 supports this.

Another example of access violation is trying to fetch opcodes from a region of memory marked not-executable, though I doubt that the MSP430 has this kind of memory protection support.

Can I write an ISR for the NMI source, and make it act like a soft reset by reinitializing the stack pointer and "recursively" calling main or something like that, so that the register and RAM (global variable) state isn't altered?

I'd look for an app note or something in the datasheet saying how to do that. Generally, jumping to main() does not work. You usually want to disable all interrupts, initialize the stack pointer, and jump to the reset vector. You may need to reset processor mode registers or control registers as well. This way, the code that sets things up before calling main() gets run.

Assuming I can't intercept the one that results in a Reset vector (which I presume also comes along with a reinitialization of all the registers?), can I at least detect at startup that it happened by checking some status bit to decide what to do about it, and if so which status register/bit would that be?

I know the Atmel AVR has a register you can read that tells you why you were reset. Not sure about the MSP430.

Related Solutions

Electronic – Msp430 Launchpad Soldering

The crystal should be close to the chip - use the pads provided.

For most applications a crystal isn't necessary as it's got a very stable on-chip oscillator.

Electronic – Timer overflow on MSP430

Not sure exactly which series MSP you're using so you'll have to look up the specifics. But the basic premise should apply across the board I think.

The following example is from the file msp430g2xx1_ta_03.c in the code snippets that can be downloaded from the TI Wiki:

#include <msp430.h>

int main(void)
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  P1DIR |= 0x01;                            // P1.0 output
  TACTL = TASSEL_2 + MC_2 + TAIE;           // SMCLK, contmode, interrupt

  _BIS_SR(LPM0_bits + GIE);                 // Enter LPM0 w/ interrupt
}

// Timer_A3 Interrupt Vector (TAIV) handler
#pragma vector=TIMERA1_VECTOR
__interrupt void Timer_A(void)
{
 switch( TAIV )
 {
   case  2: break;                          // CCR1 not used
   case  4: break;                          // CCR2 not used
   case 10: P1OUT ^= 0x01;                  // overflow
            break;
 }
}

The key is checking the TAIV register to see what caused the interrupt. Take a look at page 416 of the CC430 Family User's Guide:

TAIV Register Description

Now the code example is looking for a value of 10 in the TAIV register and the User Guide indicates that 0x0E is the overflow. So, again, check your specific chip's documentation. But the TAIV register is what will tell you what the cause of the interrupt is.

Best Answer

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Electronic – Timer overflow on MSP430

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