mirror of
https://github.com/rfivet/stm32bringup.git
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170 lines
5.5 KiB
HTML
170 lines
5.5 KiB
HTML
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<!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>1.6 The Tick</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>1.6 The Tick</h1>
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<img alt="It’s blue! It blinks! It’s the Tick!" src="img/16_tick.png">
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<p>
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In previous iteration, I made the user LED blink using an active delay
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loop. I have two issues with this implementation:
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<ul>
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<li> It’s hard to control the delay timing accurately
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</ul><ul>
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<li> Active loops are not cool
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</ul>
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So I am going to call in some serious reinforcement, which means one of
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the Arm Core Peripherals: the System Tick.
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<p>
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What the System Tick does is very similar to my active delay loop as
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can be seen from the following pseudo-code.
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<pre>
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while( enabled) {
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if( --current_value == 0) {
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current_value = reload_value ;
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countflag = true ;
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if( interrupt_enabled)
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SysTick_Handler() ;
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}
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}
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</pre>
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It’s an auto decremented counter that reloads and sets a flag when
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reaching zero. It can trigger a system interrupt if requested to. Its
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default clock is the processor clock and can be switched to external
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clock. Details can be found in the Programming Manual as this is part of
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Arm Core.
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<h2>Code, build and test</h2>
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I copy <b>blink.c</b> into <b>ledtick.c</b> to make the following
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modifications:
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<ul>
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<li> Expand the interrupt vector to make room for all 15 System Exceptions
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from <b>Reset_Handler</b> to <b>SysTick_Handler</b>.
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</ul><ul>
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<li> Introduce the <b>SysTick</b> core peripheral to the compiler, using
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pre-processor macroes to give the location of SysTick registers. As this
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is a core peripheral, it’s in a different address space than the
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peripherals I have seen so far.
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</ul><ul>
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<li> Start the <b>Reset_Handler</b> by initializing and enabling the System
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Tick. The reload value register is initialized with a constant based on
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the internal clock value divided by 8. As I want one tick per second and
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the default internal clock is 8 MHz, SysTick will have to count one
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million steps, from 999999 to 0. So the reload value is 999999.
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</ul><ul>
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<li> Create the <b>SysTick_Handler</b> interrupt routine which toggles GPIO
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B1.
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</ul><ul>
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<li> Replace the previous active delay loop by a <b>cool</b> idle loop.
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Instead of doing nothing actively, I instruct the processor to wait for
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interrupt, which means it will suspend until waked up by the SysTick.
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This is one way to lower power consumption, besides, waiting for SysTick
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interrupt is really the only thing left to do.
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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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void SysTick_Handler( void) ;
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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[ 16] __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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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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SysTick_Handler
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} ;
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#define SYSTICK ((volatile long *) 0xE000E010)
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#define SYSTICK_CSR SYSTICK[ 0]
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#define SYSTICK_RVR SYSTICK[ 1]
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#define SYSTICK_CVR SYSTICK[ 2]
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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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/* By default SYSCLK == HSI [8MHZ] */
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/* SYSTICK */
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SYSTICK_RVR = 1000000 - 1 ; /* HBA / 8 */
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SYSTICK_CVR = 0 ;
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SYSTICK_CSR = 3 ; /* HBA / 8, Interrupt ON, Enable */
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/* SysTick_Handler will execute every 1s from now on */
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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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for( ;;)
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__asm( "WFI") ; /* Wait for interrupt */
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}
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void SysTick_Handler( void) {
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GPIOB_ODR ^= 1 << 1 ; /* Toggle PB1 (User LED) */
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}
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</pre>
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I didn’t initialize the GPIO B before enabling the SysTick as I have a
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whole second before the first interrupt will tick in.
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<p>
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Build is straightforward.
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<pre>
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$ make ledtick.hex
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ledtick.elf
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text data bss dec hex filename
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148 0 0 148 94 ledtick.elf
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ledtick.hex
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rm ledtick.o ledtick.elf
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</pre>
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If I compare with blink.hex, 56 bytes of the 80 bytes code increase are
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due to the expansion of the interrupt vector.
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<p>
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Once flashed in the board I can see that the LED changes state every second.
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<h2>Checkpoint</h2>
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I now have the foundation for timing and a first taste of shifting
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execution between the main loop and an interrupt routine.
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<p>
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Code size has been growing steadily since the first bootstrap. On the
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other hand, except for the stack, I have not used RAM memory so far.
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<pre>
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│ │ text │ data │ bss │
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├────────┼──────┼───────┼───────┤
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│<b>boot</b> │ 10 │ 0 │ 0 │
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│<b>ledon</b> │ 40 │ 0 │ 0 │
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│<b>blink</b> │ 68 │ 0 │ 0 │
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│<b>ledtick</b> │ 148 │ 0 │ 0 │
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</pre>
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<a href="17_cstartup.html">Next</a>, I will focus on RAM initialization.
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<hr>© 2020-2024 Renaud Fivet
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</body>
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</html>
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