ESP8266 Design Notes

ESP8266 Espruino Port Design Notes

This page contains a number of random design notes for the esp8266 port of espruino.

Flash map and access

The esp8266 modules come in a number of forms with varying flash chip sizes. The flash layout for each size is slightly different. There are a number of random tidbits to know about flash layout:

• There are two binary formats in use: the non-OTA and the OTA update format. The non-OTA has no 2nd stage bootloader, starts at 0x0000 in flash, and has two binary blobs: one for data and one for instructions (+ read-only data). The OTA format has a 2nd stage bootloader at 0x0000 and then a single binary blob for the first firmware image at 0x1000. There is a 2nd binary blob up after the first one to provide the two firmware images necessary for a safe OTA update. All this is described in the Espressif IOT SDK User Manual.
• The hardware can memory-map 1MBytes of flash, but it has to be read on word (4-byte) boundaries, it cannot be written as far as I know
• Every memory map has a 16KB "user param" area that is reserved for applications to store non-volatile settings and such. This is used as the "save to flash" area in Espruino.
• Every memory maps also has a 16KB "system param" area in which the SDK stores settings, including RF parameters and wifi settings

Flash size	FW size	FW#1 start	FW #2 start	Save to flash	System param	SPIFFs	Free
512KB	480KB	0x000000	N/A	0x78000	0x7C000	N/A	N/A
1MB	492KB	0x001000	0x81000	0x7C000	0xFC000	N/A	0x80000 (4KB)
2MB	492KB	0x001000	0x81000	0x7C000	0xFC000	0x100000 (1MB)	0x80000 (4KB)
4MB	492KB	0x001000	0x81000	0x7C000	0xFC000	0x100000 (3MB)	0x80000 (4KB)