[GitHub] alvarop opened a new pull request #996: WIP: hal_nvreg (Retained register interface)

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alvarop opened a new pull request #996: WIP: hal_nvreg (Retained register interface)
URL: https://github.com/apache/mynewt-core/pull/996
 
 
   This is a first attempt at a hal interface to retained registers. (And added support for serial bootloader to use it to enter bootloader on reset if a specific value is saved in one of them)
   
   Various microcontrollers have a few registers that survive a soft reset. This interface exposes them (if available) via hal_nvreg (Happy to change the name if we can think of something better).
   I was thinking nv for nonvolatile, which isn't always true since they will get cleared after hard reset (in some devices). Another options was hal_retained_reg, which is a bit long, but also works.
   
   ## hal_nvreg functions
   * `void hal_nvreg_write(unsigned int reg, uint32_t val);`
     * Stores value to retained register at index <reg>
   * `uint32_t hal_nvreg_read(unsigned int reg);`
     * Returns value of retained register at index <reg>
   * `unsigned int hal_nvreg_get_num_regs();`
     * Different devices might have more, less, or no retained registers. This will return how many there are.
   * `unsigned int hal_nvreg_get_reg_width();`
     * Since not all registers are 32 bits, this function allows for querying the actual stored size
   
   ## boot_serial modifications
   Added an optional check for a specific value if **MYNEWT_VAL(BOOT_SERIAL_NVREG_INDEX) != -1**.
   
   If the value stored in retained register <**MYNEWT_VAL(BOOT_SERIAL_NVREG_INDEX)** > matches  <**MYNEWT_VAL(BOOT_SERIAL_NVREG_MAGIC)**>, then the serial bootloader starts instead of continuing to run the main application.
   
   ## Serial Bootloader Example
   I wrote a small example app to test the serial bootloader here: https://github.com/alvarop/mynewt_bootloader_app

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