Software Reset for Atmel AVR MCUs

______________________________________________

Section 1: The Problem
Section 2: Beware of the [watch] Dog!!
Section 3: The Code
______________________________________________


Despite the fact that the content of this post is targeted to AVR processors, everything explained here is valid for any other processor from any other manufacturers.

Section 1: The Problem

Well... there are no many words to say about this issue... just some processors have available some instructions to force a system RESET by software, and other processors do not.

Then, what can we do if we need to force a system reset and do not have an instruction to do it?

Ok, we are not discovering the fire. The best and simplest way to do this task is by forcing a condition for a peripheral, or system module that generates the system reset for us.

In most cases (and also for our case [AVR processors]), the simplest way is using the Watchdog Timer module to generate a system reset for us.

Basically, we will implement a function (i.e. void mySoftwareReset(void)) that performs the following tasks:

1. Configures Watchdog timer module to generate a system reset on timer overflow.

2. Enables and/or starts the Watchdog Timer.

3. Performs an endless loop (awaiting for watchdog overflow).


This is as simple as that.



Section 2: Beware of the [watch] Dog!!

Before going through the code just a very simple (yet mandatory) trick must be considered to avoid putting your processor in an endless reset loop...

You must keep the following two issues in mind:

1. After system reset caused by watchdog timer, the watchdog restarts counting and will generate another system reset on next overflow.

2. After system reset, MCU's startup code performs some tasks after calling your main firmware code, like initializing stacks, initializing .bss sections, moving some code to RAM (if it was specified in your linker file), etc. All these tasks require some computation time, of course.


If you set a very short watchdog timer period to force a system reset, and this period is smaller than system's startup code execution time, your processor's watchdog will overflow (and generate another system reset) before initialization process ends, so you will leave your system in a continuous and endless reset loop.

So remember, setting up your watchdog timer period long enough to let system execute startup code and watchdog stop/disable code once the system has been reset.



Section 3: The Code

In this section you have available some implementations of funtions performing software reset for some AVR processors.

Software reset code for Atmel AVR8 processors (includes ATtiny and ATmega)...

	/*
	* IMPORTANT NOTE:
	* The code below works for AVR8 processors (ATtiny and ATmega)
	* 'WDCE' bitfiel into WDTCSR register is present or missing based on target MCU,
	* but it do not affect the operation of the code below.
	*/
	//Some register definitions
	#ifndef WDTCSR
	#define WDTCSR (*((unsigned char*)0x21))
	#endif // WDTCSR defined
	//Predefined prescaler timeout flags.
	#define AVR8_WDT_PRESCALER_16MS ((unsigned char)0x00)
	#define AVR8_WDT_PRESCALER_32MS ((unsigned char)0x01)
	#define AVR8_WDT_PRESCALER_64MS ((unsigned char)0x02)
	#define AVR8_WDT_PRESCALER_125MS ((unsigned char)0x03)
	#define AVR8_WDT_PRESCALER_250MS ((unsigned char)0x04)
	#define AVR8_WDT_PRESCALER_500MS ((unsigned char)0x05)
	#define AVR8_WDT_PRESCALER_1S ((unsigned char)0x06)
	#define AVR8_WDT_PRESCALER_2S ((unsigned char)0x07)
	#define AVR8_WDT_PRESCALER_4S ((unsigned char)0x20)
	#define AVR8_WDT_PRESCALER_8S ((unsigned char)0x21)
	void myAVR8SoftwareReset(void)
	{
	//Configure Watchdog as...
	// WDTCSR.WDIF = 1 (just to clear WDT interrupt flag if set)
	// WDTCSR.WDIE = 0 (WDT interrupt disabled, so WDT generates system reset on overflow)
	// WDTCSR.WDE = 1 (enable WDT)
	// WDTCSR.WDCE = 1 (If this bit is present, we need set it to perform changes in WDTCSR register contents)
	// WDTCSR.WDP3_0 = (one of the prefedined prescalers above, as you want)
	WDTCSR = ((unsigned char)0x98 | AVR8_WDT_PRESCALER_500MS); //In this case we established 500ms timeout period.
	//Endless loop until watchdog overflow.
	while(1) {}
	}
	//And later, in your main function...
	int main(void)
	{
	// ... Variable declarations ...
	//First of all disable watchdog.
	WDTCSR = (unsigned char)0x10; //WDTCSR.WDE=0 and WDTCSR.WDCE=1 (if present) to disable watchdog
	// ... continue with your program code ...
	}

view raw AVR8_software_reset.c hosted with ❤ by GitHub

Software reset code for Atmel AVR32 processors...

	//Some register definitions
	#ifndef WDTCTRL
	#define WDTCTRL (*((unsigned long*)0xFFFF0D30))
	#endif // WDTCTRL defined
	#ifndef WDTCLR
	#define WDTCLR (*((unsigned long*)0xFFFF0D34))
	#endif // WDTCLR defined
	void myAvr32SoftwareReset(void)
	{
	//In AVR32, WDT module is clocked from internam 115KHz RC oscillator.
	//We use PSEL field in WDTCTRL register to set a timeout period of 2^(PSEL+1) / 115000 seconds.
	//In the code below we set a timeout period of 65536/115000 = 0.567 seconds
	//Let's configure Watchdog as...
	// WDTCTRL.KEY = [0x55, 0xAA]
	// WDTCTRL.PSEL = 15 (timeout period to 2^16 / 115000 => 65536/115000 seconds => 0.567 seconds
	// WDTCTRL.EN = 1 (enable WDT)
	WDTCTRL = 0x55000F01;
	WDTCTRL = 0xAA000F01;
	//Endless loop until watchdog overflow.
	while(1) {}
	}
	//And later, in your main function...
	int main(void)
	{
	// ... Variable declarations ...
	//Reset watchdog.
	WDTCLR = 0x0000000f; //Any value is valid.
	//Disable watchdog by setting EN field to '0'
	WDTCTRL = 0x55000F00;
	WDTCTRL = 0xAA000F00;
	// ... continue with your program code ...
	}

view raw AVR32_software_reset.c hosted with ❤ by GitHub