14 LED Demo Application

The LED demo application (pic32cmsg_led_app) is a TrustZone dual-world firmware that demonstrates the PIC32CMSG00 secure boot flow. It exercises VSS keystore reading, PUF key unwrap/verification, and basic GPIO control across the TrustZone boundary.

The application is distributed as two .hex variants, both containing the Secure and Non-Secure images ready to be programmed alongside the FWMD and VSS data:

• pic32cmsg_led_app_dice.X.production.unified.hex — DICE enabled (CDI and FW Hash: read values and report the result of the read operation)
• pic32cmsg_led_app_nodice.X.production.unified.hex — DICE disabled (standard demo)

Both are built from the same source code with different fuse configurations (BOOT_FLAG_DICEDIS = CLEAR vs. SET).

1. Reset: Cortex-M23 loads Secure MSP from 0x00000000, jumps to Secure Reset_Handler.
2. Secure main():
   • DICE: tram_read_dice_values() - reads CDI and FW Hash from TRAM (before SYS_Initialize, to preserve TRAM contents)
   • Initializes PORT, CLOCK, EVSYS, TRAM, SYSTICK, PM, SERCOM4, NVIC
   • Prints boot banner to serial console
   • Prints DICE result status (success, disabled, or error)
   • Registers SysTick callback - toggles LED1 on every tick
   • Starts SysTick timer
   • Reads Non-Secure MSP from TZ_START_NS (0x0C040000)
   • Sets Non-Secure MSP, reads NS Reset_Handler, jumps to it
3. Non-Secure main():
   • Calls SYS_Initialize() - BSP_Initialize (LED0 off) + APP_Initialize
   • Enters infinite loop calling SYS_Tasks() - APP_Tasks()
4. APP_Tasks() state machine:
   • APP_STATE_DUMP_KEYS: Calls secure_vss_dump_keys() (NSC)
   • APP_STATE_VERIFY_KEYS: Calls secure_vss_verify_puf_keys() (NSC)
   • APP_STATE_SERVICE_TASKS: Polls SW0 - LED0 on/off (loops forever)

The application supports DICE, which provides a hardware-rooted device identity based on firmware measurement. When enabled via fuses, the Boot ROM computes a CDI and stores it in TRAM for the secure application to retrieve.

How it works:

1. The Boot ROM reads the DICEDIS fuse. If clear (0), DICE is enabled.
2. The PUF generates a Unique Device Secret (UDS).
3. The Boot ROM computes: CDI = HMAC-SHA256(UDS, FW_measurement).
4. CDI (32 bytes) is stored in TRAM at the index configured by DICE_CDI_INDEX fuse.
5. FW Hash (32 bytes) is stored in TRAM at the index configured by DICE_FW_HASH_INDEX fuse.
6. Boot ROM sets DSU_BCC[1] = 0x00000004 to signal successful boot completion.
7. The secure application reads CDI and FW Hash from TRAM before SYS_Initialize().

Fuse configuration:

TRAM data format:

Console output (DICE enabled and successful):

[INFO] Retrieved CDI and FW Hash successfully 
========================================

Console output (DICE disabled):

[INFO] DICE is disabled 
========================================

Error conditions:

• [ERROR] Boot sequence not complete - DSU_BCC[1] != 0x04; Boot ROM did not finish DICE computation
• [ERROR] DICE enabled but CDI of FW Hash index invalid - Fuse indices are 0x7F (unconfigured)

All sensitive operations are performed entirely within the Secure world. The Non-Secure application calls them via veneer functions:

/* Print a message through secure UART (SERCOM4) */
void secure_print_msg(const char *msg);

/* Read and print the VSS keystore inventory to serial console */
void secure_vss_dump_keys(void);

/* Unwrap PUF key codes, compare against originals, print PASS/FAIL */
void secure_vss_verify_puf_keys(void);

Security property: No key material ever crosses the TrustZone boundary. All data is printed to the serial console directly from the Secure world.

Terminal settings: 115200 baud, 8-N-1, no flow control.

The output differs between the DICE and no-DICE variants only in the status line after the secure banner. The example below shows the DICE variant. For the no-DICE variant, the line [INFO] Retrieved CDI and FW Hash successfully is replaced by [INFO] DICE is disabled.

After this output, the system enters Idle mode:

• LED1 continues blinking (SysTick ISR in Secure world)
• LED0 responds to physical button press (SW0)
• No further serial output unless an error occurs