Added legacy blog articles

Author: guyzmo

content/code/on/AVR_bootloaders.markdown

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+---
+type: post
+categories: code
+header_background: /img/atmega32u4_closeup.jpeg
+tags: [ bootloader, avr ]
+title: "AVR bootloaders: reboot into bootloader"
+date: 2014-01-15T10:42:53+02:00
+summary: "I needed to tweak the bootloader to get a working reboot into bootloader feature. With help from the AVRFreaks community, I thought it'd be a good idea to share my experience working with AVR bootloaders."
+lang: english
+logo: "/img/avr.png"
+aliases:
+ - /code/on/avr-for-ble-and-ios/
+---
+
+When dealing with firmware updates, you need to make the AVR reboot into bootloader. And
+there are lots of solution when you try to do that:
+
+ - either you force a reboot using the 'reset' button, and try to launch the avrdude command
+   just 42ms after hitting the switch, or
+ - you can use the not always wired up and not always working properly DTR line pushed down, or
+ - you can wire a pin to the reset button — which is the worst solution, as avr pins are in an undefined state
+    at startup time) or
+ - you can use the watchdog reset to reboot.
+
+My preferred solution would be the DTR line pulled down, though it's mostly an electronic
+solution and it does not work with newer atmega USB MCUs. So, let's have fun hacking the
+bootloader.
+
+As I hacked this on the stk500 bootloader from Peter Fleury, I'm going to show the modifications
+based on that bootloader, which is known to work on all AVR from the atmega168 to the 2560. But
+that kind of hack could be easily patched on optiboot, the newer smaller (less than 1k) bootloader.
+
+In that latest version of the firmware, patched by Mark Sproul, he fixed an issue with the watchdog
+timer, which we can thank him for, as we won't have to do write it. Indeed, when you set up a watchdog
+in your firmware for something like 1 second, that for a reason your code gets reboot, and then you
+have a 2 seconds timeout in bootloader mode, the watchdog will get triggered during boot time and
+won't be reset unless you hard reset the AVR.
+
+So his code in the `_FIX_ISSUE_181_` preprocessor block fixes that:
+
+	uint8_t	mcuStatusReg;
+	mcuStatusReg	=	MCUSR;
+
+	__asm__ __volatile__ ("cli");
+	__asm__ __volatile__ ("wdr");
+	MCUSR	=	0;
+	WDTCSR	|=	_BV(WDCE) | _BV(WDE);
+	WDTCSR	=	0;
+	__asm__ __volatile__ ("sei");
+	// check if WDT generated the reset, if so, go straight to app
+    if (mcuStatusReg & _BV(WDRF))
+    {
+        app_start();
+    }
+
+This code stores the reboot status register in `mcuStatusReg`, clears the watchdog reset
+registers, and if a watchdog caused the reboot, jump straight to address `0x00` (which
+`app_start()` points to).
+
+Though the only minor modification we need to do here is:
+
+	if (mcuStatusReg & _BV(WDRF))
+	{
+        boot_timeout =  200000; // sixteen LED blinks
+	} else {
+        boot_timeout =  50000; // four LED blinks
+    }
+
+So there's a really long timeout at boot time in order for the flashing to be done. 
+
+And voila `\o/`
+
+Don't forget to merge the bootloader with your firmware before uploading using your
+favorite flasher and now you can reboot into bootloader using the following code in
+your firmware:
+
+    #include <avr/wdt.h>
+
+    void reboot_into_bootloader(void) {
+        wdt_enable(WDTO_15MS);
+        for(;;);
+    }
+