Hi,
We notice the startup time from power up can be affected by setting one GPIOB pin to input. The startup time here refers to the time from SystemInit() until main(). Basically it contains scatterload and __rt_entry to my understanding.
Specifically, the sample code below, built with ARM Compiler 6.16 Tool, would demonstrate that
#define MEASURE_PIN 5
int main(void)
{
IO_SET_PIN(GPIOA, MEASURE_PIN, 1);
// Enable IWDG
IWDG->KR = 0xCCCC;
// Enable write access
IWDG->KR = 0x5555;
// Set prescaler
IWDG->PR = 0;
// Set reload
IWDG->RLR = 1;
// Wait for the registers reload
while (IWDG->SR)
{
}
// refresh the watchdog.
IWDG->KR = 0xAAAA;
while(1);
return 0;
}
void SystemInit(void)
{
// enable GPIOA clock
RCC->IOPENR |= RCC_IOPENR_GPIOAEN | RCC_IOPENR_GPIOBEN;
// set GPIOA pin5 to output
IO_SET_PIN_DIR(GPIOA, MEASURE_PIN, 1);
// set GPIOA pin5 to low
IO_SET_PIN(GPIOA, MEASURE_PIN, 0);
// Set GPIOB pin7 to input
// GPIOB->MODER &= 0xffff3fff;
} Basically GPIOA pin 5 is toggled to low and high for the time between SystemInit() and main() to be measured.
main() also has IWDG setup and ends with a while loop so that IWDG reset happens periodically since power up.
On the oscilloscope it looks like the screenshots below.
Respectively from the screenshots, it can be observed that the 1st startup(power up) takes 254 μs while the 2nd startup (IWDG reset) takes 132 μs.
Now if uncommenting this line GPIOB->MODER &= 0xffff3fff; and redo the measurement, we get the results below.
We can see that having the GPIOB pin 7 set to input reduces the startup time from 254 μs to 132 μs and the rest startup remains taking the same time.
Could you please confirm the following:
- What causes the differences between 1st and 2nd startup?
- Why setting GPIOB ping 7 to input would affect the startup timing?
Just in case, this is how Reset_Handler looks like in our startup file. It is basically without any customization.
; Reset handler routine
Reset_Handler PROC
EXPORT Reset_Handler [WEAK]
IMPORT __main
IMPORT SystemInit
LDR R0, =SystemInit
BLX R0
LDR R0, =__main
BX R0
ENDPAnta
On the NRST pin we have a capacitor 10kΩ pull up and a resistor 100 nF to ground, and we program the options bytes NRST_MODE to have Reset Input only.
The scope pictures are as follows:
