esp_tool is an image generator and downloader utility for the ESP8266.
The esp_tool utility is a command-line application that can generate image files for the ESP8266 by extracting information from an ELF file. It can combine multiple images into a single file either in padded mode (as similar utilities do) or in sparse mode. The advantage of having a single image is that it simplifies downloading. A padded combined image, however, can take much longer to download to the device than the sparse image. Essentially, the sparse image representation avoids having to transmit all of the pad bytes.
The esp_tool utility can also automatically extract the load address from the image name if the name contains the hexadecimal load address as it often does. For example, if one of the load image files has the name myApp_0x4c000.bin, esp_tool can extract the 0x4c000 load address from the image name. This capability simplifies the process of downloading multiple image files as it does for combining multiple images.
The C++ code for esp_tool has been compiled for Windows using both VC6 and VS12, an nmake makefile is provided. It has also been compiled and briefly tested on Debian. The Makefile also works for building a Windows application using MinGW. The code contains conditionals for Mac OS X but it has not been built and tested on that platform.
The following tables give the format of a sparse combined image file. The file begins with a four-byte header and is followed by one or more segments representing the individual image files that were combined. Each of the segments begins with an eight-byte header that is followed by the image data. Note that each image is padded with zero bytes, if necessary, to make the image size an integral multiple of 4 bytes in length.
| Byte | Name | Comment |
|---|---|---|
| 0-2 | Signature | 'e', 's', 'p' |
| 3 | Segment Count | Number of segments in the file. |
| 4-n | Segment Data | Data for one or more segments. |
| Byte | Name | Comment |
|---|---|---|
| 0-3 | Address | Little-endian load address. |
| 4-7 | Length | Little-endian image size. |
| 8-n | Image Data | Load image binary data. |