Data Sheet

11.2.2 Secure Boot
Secure Boot includes several features related to boot and firmware security. Firmware authentication and encryption do not have any
impact on host software, whereas firmware roll back prevention requires more flexibility and is managed by each customer through soft-
ware.
• Firmware authentication: The downloaded firmware is authenticated such that only Firmware provided by Silicon Labs can run in
WFM200S.
• Firmware encryption: The downloaded firmware is encrypted when generated by Silicon Labs and is decrypted inside WFM200S
during firmware download.
• Firmware roll back prevention: If a security threat is discovered, Silicon Labs has the ability to increment in its firmware an anti-roll-
back tag. This can be used by the customer to prevent the part from starting with a firmware having a tag lower than a specified one.
This mechanism is managed by each customer on a case-by-case need.
11.2.3 Secure Link (WFM200SS only)
Secure Link refers to the capability to have encrypted SPI/SDIO communication between the host and WFM200S. This feature requires
the host and WFM200S to exchange a key based on a shared secret stored on both sides nonvolatile secured memories and program-
med at the end product manufacturing stage. The encrypted interface uses a Diffie-Hellman algorithm key exchanges on a per
session/per device basis. As a result, a given link is secured uniquely on a given device, and keys are regenerated on a power cycle.
There are 3 possible cases for secure link:
• Secure link is not used: In this scenario, the part does not encrypt any communication with the host.
• Secure link is temporary enabled: Secure Link can be activated through software, with a software key which is not stored in
WFM200S. Doing this allows to assess the performance and consumption impacts of secure link. In this mode, Secure Link is ach-
ieved as long as the part is not reset. The next restart of WFM200S will make it start in Non-Secure Link mode.
• Permanent Secure Link: This mode is activated by software and the key exchanged is permanently stored in WFM200S non-volatile
memory. Once configured in this mode, WFM200S only understands host interface messages which have been encrypted with the
stored key.
Once a secure link has been established, the host can choose to only encrypt certain API messages between the host and the
WFM200S to reduce the power and latency overhead of encryption.
11.3 Startup, Sleep and Shutdown
11.3.1 Power On, Reset, and Boot
When RESETn pin is set HIGH, WFM200S is getting out of its reset mode. All supply voltages should be settled within the operational
range before the rising edge of RESETn pin. Then the boot sequence can be initiated by the host software with the following sequence:
• Some registers describing the required configuration before firmware download are written by the driver.
• The driver initiates the boot.
• The driver downloads the embedded firmware into WFM200S.
• The driver configures WFM200S upon the hardware platform and requested features with a dedicated configuration file.
11.3.2 Sleep and Snooze Modes
The sleep or snooze modes are reached when power-save mode has been enabled on the WFM200S. These modes highly reduce
power consumption while maintaining all configuration and context, so that the device can be quickly back to normal operation. A
WFM200S driver command is used to indicate that the driver wants the part to go to power-save. However it is the firmware on
WFM200S that decides when it switches into sleep mode based on Wi-Fi activity.
The part wake-up is achieved by asserting the GPIO/WUP pin.
The sleep mode requires a 32 KHz clock to be provided on LP_CLK pin.
In case a 32 KHz clock is not available, the part can be set in a snooze mode which is functionally equivalent but draws more current.
WFM200S Data Sheet
Software Reference
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