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https://github.com/rfivet/stm32bringup.git
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102 lines
3.1 KiB
HTML
102 lines
3.1 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>1.5 Blinking user LED</title>
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<link type="text/css" rel="stylesheet" href="style.css">
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</head>
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<body>
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<h1>1.5 Blinking user LED</h1>
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Turning the user LED on shows that the board is alive, making it blink
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shows that it has a pulse! A steady power LED with a pulsing user LED is
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easy to interpret.
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<p>
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As I already manage to turn the LED on, making it blink is quite
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straightforward.
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<h2>Code and build</h2>
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I make a copy of <b>ledon.c</b> into <b>blink.c</b> and modify it as follow:
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<ul>
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<li>Introduce the Output Data Register (<b>ODR</b>) of <b>GPIOB</b> peripheral
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to the compiler as <b>GPIOB_ODR</b>. This is where the pin state is programmed.
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<li>Modify the idle loop to toggle PB1 after a delay. The delay is coded
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as an active wait loop on decrementing a counter.
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</ul>
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<pre>
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/* Memory locations defined by linker script */
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extern long __StackTop ; /* &__StackTop points after end of stack */
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void Reset_Handler( void) ; /* Entry point for execution */
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/* Interrupt vector table:
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* 1 Stack Pointer reset value
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* 15 System Exceptions
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* NN Device specific Interrupts
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*/
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typedef void (*isr_p)( void) ;
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isr_p const isr_vector[ 2] __attribute__((section(".isr_vector"))) = {
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(isr_p) &__StackTop,
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/* System Exceptions */
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Reset_Handler
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} ;
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#define RCC ((volatile long *) 0x40021000)
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#define RCC_AHBENR RCC[ 5]
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#define RCC_AHBENR_IOPBEN 0x00040000 /* 18: I/O port B clock enable */
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#define GPIOB ((volatile long *) 0x48000400)
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#define GPIOB_MODER GPIOB[ 0]
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#define GPIOB_ODR GPIOB[ 5]
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void Reset_Handler( void) {
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int delay ;
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/* User LED ON */
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RCC_AHBENR |= RCC_AHBENR_IOPBEN ; /* Enable IOPB periph */
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GPIOB_MODER |= 1 << (1 * 2) ; /* PB1 Output [01], over default 00 */
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/* OTYPER Push-Pull by default */
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/* PB1 output default LOW at reset */
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/* User LED blink */
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for( ;;) {
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for( delay = 1000000 ; delay ; delay--) ; /* delay between toggling */
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GPIOB_ODR ^= 1 << 1 ; /* toggle PB1 (User LED) */
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}
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}
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</pre>
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I set the value of the delay counter at one million. By default the
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internal clock is set to 8MHz at reset, which means the delay will still
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be less than one second.
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<pre>
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$ make blink.hex
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blink.elf
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text data bss dec hex filename
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68 0 0 68 44 blink.elf
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blink.hex
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rm blink.o blink.elf
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</pre>
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<h2>Test</h2>
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As the <a href="vid/15_blink.mp4">video</a> shows, the delay is roughly
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600ms. I captured three on/off transitions in this three second video, looking
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through the frames gives me a better estimation.
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<h2>Checkpoint</h2>
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This is just a small increment on my previous step, but that’s iterative
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development in a nutshell. Also I didn't come with a reasonable value
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for the delay counter at first, it's easy to underestimate how fast
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micro-controllers are.
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<p>
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<a href="16_ledtick.html">Next</a>, interrupt driven blinking, because
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active wait delay is just not cool.
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<hr>© 2020-2024 Renaud Fivet
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</body>
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</html>
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