ESP8266 Design Notes

ESP8266 Espruino 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. Currently the number of variables is set to 1023, which uses 12KB and in addition the saved data is run-length encoded. Therefore at most 12KB of this flash area really need to be reserved. It is not known whether there's a chance to increase the number of variables at this point.
• Every memory maps also has a 16KB "system param" area in which the SDK stores settings, including RF parameters and wifi settings
• Finally there is an unused 4KB area just before the 2nd firmware in the larger memory maps, this "mirrors" the bootloader area but is not used.

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)