For initial loading, the method will work. In fact, it is similar to the method we have considered while designing the wear levelling library. The idea was to create a file system using existing tools such as mkfs.vfat, mount it, load files into it, unmount, add the correct wear levelling metadata a...

Re. doing calibration at runtime — this can be exposed as a API to applications, so that they can do it once in a while if needed. Or just use 32k xtal if time precision matters.

You may possibly have to call opendir("/spiflash/"). The fact that the form without the trailing slash doesn't work is likely a bug in VFS. Thanks for reporting.

I was thinking along the similar lines, the only remaining question is how to do this without introducing RTC_DATA variables. Currently if the application doesn't use RTC_DATA, we can power it down. Another option is to store these values in RTC_FAST_MEM, if it is kept enabled. Probably will go for ...

XTAL selection was added in 6b237de: https://github.com/espressif/esp-idf/commit/6b237de90982d1125bde4edda7c6edc0ed8ec511 So if you check out the latest version of master branch, you should have this change. But as you have correctly noted above, 26MHz should also work on earlier versions by autodet...

The explicit choice of XTAL frequency has been added to menuconfig recently, and if you use latest git version of ESP-IDF, the option should be there. 26 MHz should work with older versions with autodetect. Can you print out the contents of RTC_CNTL_STORE4_REG and RTC_CNTL_STORE5_REG and post them h...

If the column is blank for a particular GPIO, then pullup, pulldown, and input are disabled (=hi-Z).
ie=0 means that input is disabled, but that doesn't mean the output is enabled, ie and oe settings are independent. All pins have outputs disabled at and after reset.

Please check the "ESP32 Pin List" document (available from Espressif website). In the pin list table, see the two rightmost columns ("at reset", "after reset"). Notes 9 and 10 on the previous page explain the contents of these two columns

The chip does support 24MHz, and the software can be made to support 24MHz as well. However this isn't yet implemented because there wasn't sufficient demand for this. Your interest is noted and I will check about prioritizing this feature for the next release. Choice between 26 and 40 MHz in menuco...