why am I getting a precise bus fault exception (PRECISERR) on what looks like a perfectly fine aligned access (cortex-m7)

2021-01-23 04:04 Brett imported from Stackoverflow

I'm getting a HardFault that results from a forced/escalated Precise Bus Fault Exception, as indicated by the PRECISERR bit in the BFSR register, and I can't seem to figure out why it is occurring. The exception occurs from within vendor-supplied startup code that previously executed fine, and I cant see any alignment or memory-related issues.

The offending instruction is ldrlt r0, [r1], #4 on the first iteration through the loop, where the value stored in r1 is 0x00040458

The full instruction sequence is shown below, where other relevant symbols used in r2 and r3 are defined in the comments

/*     Loop to copy data from read only memory to RAM. The ranges
 *      of copy from/to are specified by following symbols evaluated in
 *      linker script.
 *      __etext: End of code section, i.e., begin of data sections to copy from.
 *      __data_start__/__data_end__: RAM address range that data should be
 *      __noncachedata_start__/__noncachedata_end__ : none cachable region
 *      copied to. Both must be aligned to 4 bytes boundary.  */

    ldr    r1, =__etext          /* equal to 0x00040458 */
    ldr    r2, =__data_start__   /* equal to 0x20000000 */
    ldr    r3, =__data_end__     /* equal to 0x20000224 */

.LC0:
    cmp     r2, r3
    ittt    lt
    ldrlt   r0, [r1], #4  /* <---- exception triggered here */
    strlt   r0, [r2], #4
    blt    .LC0

The offending address listed in BFAR is 0x00040458, which corresponds to the value in r1 and is a perfectly valid 32-bit aligned address within the ITCM region (0x0 --> 0x0007FFFF).

Not sure what else could be causing this exception if the memory access itself looks fine. The exception was introduced by expanding the .text section in my linker file, as shown below

MEMORY
{
  m_interrupts  (RX)  : ORIGIN = 0x00000000, LENGTH = 0x00000400
  m_text        (RX)  : ORIGIN = 0x00000400, LENGTH = 0x00074000  /* changed from LENGTH = 0x0003FC00 */
  m_data        (RW)  : ORIGIN = 0x20000000, LENGTH = 0x00020000
  m_data2       (RW)  : ORIGIN = 0x20200000, LENGTH = 0x00020000
}

If it isn't an alignment issue, I'm not sure what it could be? But 0x00040458 is most definitely word-aligned, as is 0x0004045C which results from the #4 offset to the ldr instruction.

Also, why is 0x0004045C not shown in BFAR, since the cortex-m7 TRM says the ldr instruction applies the offset to the target register value before the memory access occurs??

Full exception registers shown below for completeness

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}

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Edit 1

Here is the latest code that I have. This one turns on and off LED randomly. I have also disabled the Watchdog Timer:

// CONFIG1
#pragma config FOSC = ECH       // Oscillator Selection (ECH, External Clock, High Power Mode (4-32 MHz): device clock supplied to CLKIN pin)
#pragma config WDTE = OFF       // Watchdog Timer Enable (WDT disabled)
#pragma config PWRTE = OFF      // Power-up Timer Enable (PWRT disabled)
#pragma config MCLRE = ON       // MCLR Pin Function Select (MCLR/VPP pin function is MCLR)
#pragma config CP = OFF         // Flash Program Memory Code Protection (Program memory code protection is disabled)
#pragma config CPD = OFF        // Data Memory Code Protection (Data memory code protection is disabled)
#pragma config BOREN = ON       // Brown-out Reset Enable (Brown-out Reset enabled)
#pragma config CLKOUTEN = OFF   // Clock Out Enable (CLKOUT function is disabled. I/O or oscillator function on the CLKOUT pin)
#pragma config IESO = ON        // Internal/External Switchover (Internal/External Switchover mode is enabled)
#pragma config FCMEN = ON       // Fail-Safe Clock Monitor Enable (Fail-Safe Clock Monitor is enabled)

// CONFIG2
#pragma config WRT = OFF        // Flash Memory Self-Write Protection (Write protection off)
#pragma config PLLEN = ON       // PLL Enable (4x PLL enabled)
#pragma config STVREN = ON      // Stack Overflow/Underflow Reset Enable (Stack Overflow or Underflow will cause a Reset)
#pragma config BORV = LO        // Brown-out Reset Voltage Selection (Brown-out Reset Voltage (Vbor), low trip point selected.)
#pragma config LVP = ON         // Low-Voltage Programming Enable (Low-voltage programming enabled)


#include <xc.h> // include standard header file

// Definitions
#define _XTAL_FREQ  16000000        // this is used by the __delay_ms(xx) and __delay_us(xx) functions

void main() {
    // set up oscillator control register
    OSCCONbits.SPLLEN = 0; // PLL is disabled
    OSCCONbits.IRCF = 0x0F; //set OSCCON IRCF bits to select OSC frequency=16Mhz
    OSCCONbits.SCS = 0x02; //set the SCS bits to select internal oscillator block

    ANSELAbits.ANSA0 = 0; // Set to digital
    ANSELAbits.ANSA1 = 0; // Set to digital
    ANSELAbits.ANSA2 = 0; // Set to digital
    ANSELAbits.ANSA4 = 0; // Set to digital

    // Set up I/O pins
    // PORT A Assignments
    TRISAbits.TRISA0 = 0; // RA0 = Digital Voltage out
    TRISAbits.TRISA1 = 0; // RA1 = Digital Voltage out
    TRISAbits.TRISA2 = 0; // RA2 = Digital Voltage out
    TRISAbits.TRISA3 = 0; // RA3 = Digital Voltage out
    TRISAbits.TRISA4 = 1; // RA4 = Digital Voltage in
    TRISAbits.TRISA5 = 0; // RA0 = Digital Voltage out

    PORTAbits.RA0 = 0;
    PORTAbits.RA1 = 0;
    PORTAbits.RA2 = 0;
    PORTAbits.RA3 = 0;
    PORTAbits.RA4 = 0;
    PORTAbits.RA5 = 0;

    // Set up ADC
    ADCON0 = 0; // ADC is off

    for (;;) {
        __delay_ms(100); // wait 100ms
        if (PORTAbits.RA4 == 0) {
            LATAbits.LATA2 = 0;
        } else {
            LATAbits.LATA2 = 1;
        }
    }
}

Ok, here is what I observed. When the button is pressed the LED never turns off, but when the button is not pressed it randomly turns on or off.

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//  Pins available on connector P8
//
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    UART4_Init(115200);
    
    send_str("External INT Demo ready\r\nTry USART4 ...\r\n"); // to UART1
    send4_str("Send to USART4 ready?\r\n"); // to UART4

    while(1)
    {
        // just loop ...
    }
}

The USART4 ports are ID'd from the board schematic. The Virtual USB comm ports are connected to USART1 and USART4. I expected that just sending some text to USART4 would work, but nothing. I do get a very occasional garbage character which usually means the baud rate is wrong. I've looked at the UART4 register settings and it all seems correct as described in the TK32F499 user manual (455 pages).

assigning a variable using binary format - 0B causing error in ARM Keil uVision
I am workin on a project with TM4C123GH6PM micro-controller using keil uvision version 4.7. When I assign a value to a variable in binary format like the following:
```
unsigned char tmp = 0b11000011;
```
and then I build the project, the following error appears:

expected a ";"

When I change the format to hex -using 0X- or Decimal, the error disappears.

doesn't the compiler in Keil uVision support the binary format?

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