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The Silicon Labs EM35x series devices have different levels of read out protection which can be configured in the option bytes which are part of the so called Customer Information Block (CIB) at address 0x08040800. The CIB is 2048 bytes in size whereof the first eight half-words are the option bytes. The first byte of the option bytes (Option Byte 0) contains the flash read out protection configuration. A special sequence, consisting of different read / write accesses to special function registers of the MCU are required to change the value of the option bytes (examples below). For detailed information about the sequence, please refer to the reference manual of the device.

The table below describes the different read protections of Option Byte 0 - flash read protection:

Byte value Read protection level
0xA5 Disabled
!= 0xA5 Enabled


J-Flash: The sample project below enables the read out as part of the Exit steps in the J-Flash project. This is done by erasing the option bytes which results in Option Byte 0 == 0xFF --> read protection enabled.