NXP AN13094, LPC55S Application Note

AN13094

Using FreeRTOS on LPC55Sxx Series Microcontrollers with
TrustZone

Rev. 0 — January 15, 2020

by: NXP Semiconductors

Contents

1 Introduction

The LPC55Sxx series MCU is a microcontroller based on the Arm
Cortex®-M33 core, using the ARMv8-M architecture with TrustZone enabled.
LPC55S69 is one of the high-performance MCUs, including two Cortex-M33
cores, and CPU0 supports the security extension of TrustZone-M. FreeRTOS
is a lightweight embedded operating system. It has the characteristics of open
source code, portability, tailorability, and flexible scheduling strategy. It can
be easily transplanted to various embedded controllers and has been widely
used in various embedded products. This document takes an LPC55S69 as
example to describe how to use FreeRTOS in an ARMv8-M processor that
supports TrustZone.

®

2 Features of TrustZone technology

TrustZone technology has the following features:

1 Introduction......................................1

2 Features of TrustZone technology

........................................................ 1

3 Security environment configuration

........................................................ 2

3.1 TEE tool....................................... 2

3.2 Secure/Non-secure state switch

.....................................................3

4 FreeRTOS usage with TrustZone... 4

4.1 FreeRTOS example with

TrustZone in SDK v2.8.................5

4.2 A safer way to use FreeRTOS...11

5 Reference......................................12

Application Note

• Allows users to divide memory map into Secure and Non-Secure regions.

• Blocks the debugging of secure code/data when not authenticated.

• CPU includes Security Attribution Unit (SAU) as well as a duplication of NVIC, MPU, SYSTICK, core control registers, etc.
Secure/Non-Secure codes have access to their own allocated resources.

• Stack management expands from two-stack pointers in original Cortex-M, Main Stack Pointer (MSP) and Process Stack
Pointer (PSP), to four, providing the above pairs individually to both Secure and Non-Secure.

• Introduces the concept of Secure Gateway opcode to allow secure code to define a strict set of entry points into it from a
Non-secure code.

TrustZone technology address some of the following security requirements of embedded systems directly.

• Data protection

Sensitive data are stored in Secure memory spaces and are only accessed by Secure software. Only after security check or
authentication, non-secure software can access to Secure APIs providing services to the Non-secure domain.

• Firmware protection

The pre-loaded firmware is stored in Secure memories to prevent it from being reverse engineered and compromised by
malicious attacks. TrustZone technology for ARMv8-M can work with extra protection techniques. For example, device level
read-out protection, a technique that is commonly used in the industry today, can be used with TrustZone technology for
ARMv8-M to protect the completed firmware of the final product.

• Operation protection

Software for critical operations can be pre-loaded as Secure firmware and the appropriate peripherals can be configured
to permit access from the Secure state only. In this way, the operations can be protected from intrusion from the
Non-secure side.

• Secure boot

NXP Semiconductors

Security environment configuration

The Secure boot mechanism enables the confidence in the platform, as it will always boot from Secure memory.

3 Security environment configuration

This section introduces how to configure the security environment to use TrustZone technology to protect the important resources
of the system. LPC55S69 provides two levels of protection: CPU-level protection and system-level protection. TrustZone is
located inside CPU0 and belongs to CPU-level protection. In addition, LPC55S69 uses secure AHB controller to provide a layer
of system-level protection, as shown in Figure 1.

Figure 1. Two-level protection of LPC55S69

The configuration of the LPC55S69 security environment includes two parts: the configuration of TrustZone and the configuration
of the Secure AHB controller. The configuration of TrustZone is mainly the configuration of SAU.

3.1 TEE tool

The configuration of the security environment can be implemented by manually configuring the corresponding registers, or by
using NXP’s Trusted Execution Environment (TEE) tool. It is recommended that developers use TEE tool to quickly implement
the configuration of TrustZone and secure AHB controller. Figure 2 shows the GUI interface of TEE tool.

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NXP Semiconductors

Security environment configuration

Figure 2. GUI interface of TEE tool

For details on using TEE tool, see

User Guide for MCUXpresso Config Tools (Desktop)

(document GSMCUXCTUG).

3.2 Secure/Non-secure state switch

After configuring the security environment of LPC55S69, users can use some special functions in the actual project to switch
between secure and non-secure states. Here are two special functions: Non-secure Callable (NSC function/Entry function) and
Non-secure function.

• NSC function

NSC functions are the secure functions that can be called by non-secure functions. NSC function needs to be defined with
the __attribute__((cmse_nonsecure_entry)) attribute. The example is as below.

__attribute__((cmse_nonsecure_entry)) void vToggleGreenLED(void)

{

/* Toggle the on-board green LED. */

GPIO_PortToggle(GPIO, LED_PORT, (1U << GREEN_LED_PIN));

}

• Non-secure function

Non-secure functions are the functions that can be called by secure functions. Non-secure function needs to be defined with
the __attribute_((cmse_nonsecure_call)) attribute. The example is as below.

Typedef void _attribute_((cmse_nonsecure_call)) nsfunc(void);

Nsfunc *FunctionPointer;

FunctionPointer = cmse_nsfptr_create((nsfunc *) (0x21000248u));

If (cmse_is_nsfptr(FunctionPointer))

FunctionPointer();

NOTE

Non-secure functions must be called by the way of function pointers.

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NXP Semiconductors

FreeRTOS usage with TrustZone

For more details about secure/non-secure state switching, see documents on TrustZone technology for ARMv8-M Architecture.

4 FreeRTOS usage with TrustZone

This section describes how to run FreeRTOS in LPC55S69 with TrustZone enabled. Officially FreeRTOS provides FreeRTOS
examples that support TrustZone to run on LPC55S69. FreeRTOS can be downloaded from Free RTOS. The path of LPC55S69
example is

FreeRTOS/Demo/CORTEX_MPU_M33F_NXP_LPC55S69_MCUXpresso

, as shown in Figure 3.

Figure 3. LPC55S69 FreeRTOS example

FreeRTOS provide documents about this demo. The documents can be downloaded from RTOS.

NXP's LPC55S69 SDK also provides a FreeRTOS example that supports TrustZone. The project name is
the path of the example is

freertos_tzm

project is as shown in Figure 4.

/SDK_2.8.2_LPCXpresso55S69_IAR/boards/lpcxpresso55s69/rtos_examples/freertos_tzm

freertos_tzm

and

. The

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