Create isp_flashing_guide.md

docs/isp_flashing_guide.md

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+# ISP Flashing Guide
+
+If you're having trouble flashing/erasing your board, and running into cryptic error messages like any of the following:
+
+    libusb: warning [darwin_transfer_status] transfer error: timed out
+    dfu.c:844: -ETIMEDOUT: Transfer timed out, NAK 0xffffffc4 (-60)
+    atmel.c:1627: atmel_flash: flash data dfu_download failed.
+    atmel.c:1629: Expected message length of 1072, got -60.
+    atmel.c:1434: Error flashing the block: err -2.
+    ERROR
+    Memory write error, use debug for more info.
+    commands.c:360: Error writing memory data. (err -4)
+
+    dfu.c:844: -EPIPE: a) Babble detect or b) Endpoint stalled 0xffffffe0 (-32)
+    Device is write protected.
+    dfu.c:252: dfu_clear_status( 0x7fff4fc2ea80 )
+    atmel.c:1434: Error flashing the block: err -2.
+    ERROR
+    Memory write error, use debug for more info.
+    commands.c:360: Error writing memory data. (err -4)    
+    
+You're likely going to need to ISP flash your board/device to get it working again. Luckily, this process is pretty straight-forward, provided you have any extra programmable keyboard, Arduino, or Teensy 2.0/Teensy 2.0++. There are also dedicated ISP flashers available for this, but most cost >$15, and it's assumed that if you are googling this error, this is the first you've heard about ISP flashing, and don't have one readily available (whereas you might have some other AVR board). __We'll be using a Teensy 2.0 with Windows 10 in this guide__ - if you are comfortable doing this on another system, please consider editing this guide and contributing those instructions!
+
+## Software needed
+
+* [The Arduino IDE](https://www.arduino.cc/en/Main/Software)
+* [Teensyduino](https://www.pjrc.com/teensy/td_download.html) (if you're using a Teensy)
+* [WinAVR](http://www.ladyada.net/learn/avr/setup-win.html) (Windows)
+
+## Wiring
+
+This is pretty straight-forward - we'll be connecting like-things to like-things in the following manner:
+
+    Flasher B0  <-> Keyboard RESET
+    Flasher B1  <-> Keyboard B1 (SCLK)
+    Flasher B2  <-> Keyboard B2 (MOSI)
+    Flasher B3  <-> Keyboard B3 (MISO)
+    Flasher VCC <-> Keyboard VCC
+    Flasher GND <-> Keyboard GND
+    
+## The ISP firmware
+
+Make sure your keyboard is unplugged from any device, and plug in your Teensy.
+
+1. Run Arduino after you have everything installed
+2. Select `Tools > Board * > Teensy 2.0`
+3. Click `File > Examples > 11.ArduinoISP > ArduinoISP`
+
+Then scroll down until you see something that looks like this block of code:
+
+    // Configure which pins to use:
+
+    // The standard pin configuration.
+    #ifndef ARDUINO_HOODLOADER2 
+
+    #define RESET     0  // Use 0 (B0) instead of 10
+    #define LED_HB    11 // Use 11 (LED on the Teensy 2.0)
+    #define LED_ERR   8  // This won't be used unless you have an LED hooked-up to 8 (D3)
+    #define LED_PMODE 7  // This won't be used unless you have an LED hooked-up to 7 (D2)
+    
+And make the changes in the last four lines. If you're using something besides the Teenys 2.0, you'll want to choose something else that makes sense for `LED_HB`. We define `RESET` as `0`/`B0` because that's what's close - if you want to use another pin for some reason, [you can use the pinouts to choose something else](https://www.pjrc.com/teensy/pinout.html). 
+
+Once you've made your changes, you can click the Upload button (right arrow), which will open up the Teensy flasher app - you'll need to press the reset button on the Teensy the first time, but after that, it's automatic (you shouldn't be flashing this more than once, though). Once flashed, the orange LED on the Teensy will flash on and off, indicating it's ready for some action. 
+
+## The .hex file
+
+Before flashing your firmware, you're going to need to and do a little preparation. We'll be appending [this bootloader (also a .hex file)](https://github.com/qmk/qmk_firmware/blob/master/util/bootloader.hex) to the end of our firmware by opening the original .hex file in a text editor, and removing the last line, which should be `:00000001FF` (this is an EOF message). After that's been removed, copy the entire bootloader's contents and paste it at the end of the original file, and save it. 
+
+It's possible to use other bootloaders here in the same way, but __you need a bootloader__, otherwise you'll have to ISP to write new firmware to your keyboard.
+
+## Flashing your firmware
+
+Make sure your keyboard is unplugged from any device, and plug in your Teensy.
+
+Open `cmd` and navigate to your where your modified .hex file is. We'll pretend this file is called `main.hex`, and that your Teensy 2.0 is on the `COM3` port - if you're unsure, you can open your Device Manager, and look for `Ports > USB Serial Device`. Use that COM port here. You can confirm it's the right port with:
+
+    avrdude -c avrisp -P COM3 -p atmega32u4
+    
+and you should get something like the following output:
+
+    avrdude: AVR device initialized and ready to accept instructions
+
+    Reading | ################################################## | 100% 0.02s
+
+    avrdude: Device signature = 0x1e9587
+
+    avrdude: safemode: Fuses OK
+
+    avrdude done.  Thank you.
+
+Since our keyboard uses an `atmega32u4` (common), that is the chip we'll specify. This is the full command:
+
+     avrdude -c avrisp -P COM3 -p atmega32u4 -U flash:w:main.hex:i
+     
+You should see a couple of progress bars, then you should see:
+
+    avrdude: verifying ...
+    avrdude: 32768 bytes of flash verified
+
+    avrdude: safemode: Fuses OK
+
+    avrdude done.  Thank you.
+    
+Which means everything should be ok! Your board may restart automatically, otherwise, unplug your Teensy and plug in your keyboard - you can leave your Teensy wired to your keyboard while testing things, but it's recommended that you desolder it/remove the wiring once you're sure everything works.
+
+If you have any questions/problems, feel free to [open an issue](https://github.com/qmk/qmk_firmware/issues/new)!