Chapter 254: ESP32-C3 Architecture (RISC-V Based)

Chapter Objectives

By the end of this chapter, you will be able to:

• Describe the single-core RISC-V CPU architecture of the ESP32-C3.
• Explain the significance of Espressif adopting the RISC-V open standard.
• Detail the C3’s connectivity features, including Wi-Fi 4 and Bluetooth 5 LE.
• Understand the C3’s target applications as a secure, cost-effective IoT node.
• Recognize the programming implications of moving from Xtensa to RISC-V.
• Identify the C3’s key security features and I/O limitations.

Introduction

In our journey through the ESP32 variants, we now encounter a pivotal moment in the family’s evolution: the ESP32-C3. This System-on-Chip marks Espressif’s first major foray into the world of RISC-V, an open-standard instruction set architecture (ISA). Unlike the proprietary Xtensa cores used in the original, S2, and S3 variants, RISC-V is a free and open standard, fostering a global community of innovation and collaboration.

The ESP32-C3 is not designed to be a powerhouse like the S3. Instead, it’s a highly optimized, secure, and cost-effective solution aimed at the vast number of applications that require reliable connectivity and robust security without the need for multiple cores or a vast peripheral set. It is often seen as the modern, more secure successor to the legendary ESP8266, providing a low-cost entry point into the ESP32 ecosystem with the latest connectivity standards. This chapter will explore the architecture of this efficient little chip and what its RISC-V core means for you as a developer.

Theory

The ESP32-C3’s design philosophy is centered on efficiency, security, and value. This is reflected in every aspect of its architecture.

1. CPU Architecture: A Shift to RISC-V

The single most important feature of the C3 is its processor. It moves away from the Xtensa architecture entirely.

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        B --> C{<b>Black Box</b><br>Limited Customization};
        C --> D[ESP32 / S2 / S3];
    end

    subgraph "Open-Standard ISA (RISC-V)"
        E{<b>Open Specification</b><br>Managed by RISC-V Int'l} --> F(Espressif Designs/Extends Core);
        F --> G{<b>Full Control</b><br>Flexibility & No Licensing Fees};
        G --> H[ESP32-C3 / C6 / H2];
    end
    
    subgraph "Benefits for Ecosystem & Espressif"
        I(Innovation & Collaboration)
        J(No Vendor Lock-in)
        K(Growing Toolchain Support)
    end

    G -.-> I;
    G -.-> J;
    G -.-> K;

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    class A,B,C,D prop;
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• Single-Core RISC-V (RV32IMC) CPU: The C3 is built around a 32-bit single-core RISC-V CPU that can run at up to 160 MHz. The “RV32IMC” designation specifies its capabilities:
  • RV32I: The base integer instruction set.
  • M: Standard extension for integer multiplication and division.
  • C: Standard extension for compressed instructions, which reduces code size.
• Why RISC-V? Adopting an open-standard ISA means Espressif is not dependent on a single third-party core designer. It allows them to freely customize and extend the core for future designs and benefits from a growing global ecosystem of tools, compilers, and expertise. For the developer, while high-level ESP-IDF code remains the same, this architectural change is fundamental.

2. Connectivity: Modern and Efficient

The C3 is equipped with the two most important wireless protocols for modern IoT devices.

• Wi-Fi 4 (802.11 b/g/n): Provides standard, reliable Wi-Fi connectivity.
• Bluetooth 5 (LE): The C3 includes a full-featured Bluetooth 5 Low Energy radio. This is a significant advantage over the ESP8266 and makes it perfect for applications involving device provisioning, beacons, or communication with low-power sensors and smartphones. It supports features like extended advertising and the 2Mbps PHY for higher throughput.

3. Memory and I/O: Lean and Focused

To achieve its cost target, the C3 has a more constrained set of memory and I/O resources.

• Memory: It contains 400 KB of on-chip SRAM and 384 KB of ROM. This is ample for a wide range of IoT applications that are not performing memory-intensive tasks like graphics processing.
• GPIO: The C3 has a much smaller pin count than the S-series, offering up to 22 GPIO pins. This is a critical consideration during hardware design and makes it unsuitable for products requiring a large number of wired connections.
• No Native USB: Like the original ESP32, the C3 does not have a native USB OTG controller. It relies on an external USB-to-UART bridge chip on the development board for programming and serial logging.

4. A Strong Security Foundation

Despite being a low-cost variant, the C3 does not compromise on security. It inherits many of the advanced security features from the more expensive S-series.

• Secure Boot: Ensures that the device only boots authentic, signed firmware.
• Flash Encryption: Protects the application code stored in external flash from being read out.
• Digital Signature (DS) Peripheral: The hardware accelerator for creating secure digital signatures, allowing the device to prove its identity to a cloud service.
• World Controller: A peripheral that allows for creating isolated execution environments for enhanced security.

Practical Examples

The beauty of the ESP-IDF is how it abstracts the underlying CPU architecture. A “Hello World” program for the C3 looks identical to one for an Xtensa-based chip at the C-code level. The key is in the project configuration and the toolchain that compiles the code.

Example 1: RISC-V “Hello World”

This example confirms we are running on a C3 and demonstrates that our familiar ESP-IDF functions work perfectly on the new architecture.

1. Code: Create a new project in VS Code, ensuring you select ESP32-C3 as the target. The main.c file is straightforward.

#include <stdio.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "esp_log.h"
#include "esp_chip_info.h"

static const char *TAG = "C3_TEST";

void app_main(void)
{
    ESP_LOGI(TAG, "Hello from the RISC-V World!");

    // This code is portable across ESP32 variants
    esp_chip_info_t chip_info;
    esp_chip_info(&chip_info);

    ESP_LOGI(TAG, "This is an %s chip with %d CPU core(s), WiFi%s%s, ",
             CONFIG_IDF_TARGET,
             chip_info.cores,
             (chip_info.features & CHIP_FEATURE_WIFI_BGN) ? "/b/g/n" : "",
             (chip_info.features & CHIP_FEATURE_WIFI_AX) ? "/ax" : "");

    ESP_LOGI(TAG, "silicon revision %d, ", chip_info.revision);

    ESP_LOGI(TAG, "%dMB %s flash", spi_flash_get_chip_size() / (1024 * 1024),
             (chip_info.features & CHIP_FEATURE_EMB_FLASH) ? "embedded" : "external");
    
    ESP_LOGI(TAG, "Minimum free heap size: %d bytes", esp_get_minimum_free_heap_size());
}

2. Build and Flash:

1. Connect your ESP32-C3 board.
2. Use the standard “ESP-IDF: Build, Flash and Monitor” command. Behind the scenes, VS Code will invoke the RISC-V toolchain (compilers, assemblers, and linkers) instead of the Xtensa toolchain.

3. Observe:

The monitor output will clearly identify the chip as an ESP32-C3 with a single RISC-V core.

I (291) C3_TEST: Hello from the RISC-V World!
I (291) C3_TEST: This is an esp32c3 chip with 1 CPU core(s), WiFi/b/g/n,
I (301) C3_TEST: silicon revision 3,
I (311) C3_TEST: 4MB external flash
I (311) C3_TEST: Minimum free heap size: 345678 bytes

Variant Notes

The ESP32-C3 establishes a new category in the Espressif lineup, focused on high-volume, cost-sensitive applications.

Common Mistakes & Troubleshooting Tips

Exercises

1. Code Porting Challenge: Take the dual-core parallel processing example from Chapter 251. Your task is to modify it so that it compiles and runs correctly on the ESP32-C3. This will involve removing the second task and all calls to xTaskCreatePinnedToCore. This demonstrates how to adapt multi-core code for a single-core environment.
2. Secure BLE Beacon: Write an application that turns the ESP32-C3 into a simple BLE beacon. It should advertise a device name like “C3_Secure_Sensor” and a manufacturer-specific data field containing a simulated temperature reading that updates every 5 seconds. This is a classic C3 application.
3. GPIO Budgeting: Design a schematic or pinout table for a small weather station project using an ESP32-C3. The project requires:
   • An I2C connection for a BME280 sensor (SDA, SCL).
   • A UART connection for a GPS module (TX, RX).
   • A single GPIO for a status LED.
   • A single GPIO for a user input button.Calculate the total number of GPIOs needed and assign them to valid pins on the C3. This exercise highlights the importance of resource planning.

Summary

• The ESP32-C3 represents a major architectural shift to the open-standard RISC-V ISA, using a single-core RV32IMC CPU.
• It is optimized for cost-effective, high-volume IoT applications.
• It combines modern connectivity with Wi-Fi 4 and Bluetooth 5 LE.
• Despite its low cost, it features a strong security suite, including Secure Boot and Flash Encryption.
• It has significant I/O constraints compared to other variants, with only ~22 GPIO pins and no native USB.
• The ESP-IDF abstracts away most architectural differences, allowing for a smooth development experience, but low-level code is not portable from Xtensa.

Further Reading

• ESP32-C3 Technical Reference Manual: The official and complete hardware documentation.
  • ESP32-C3 Technical Reference Manual (PDF)
• Espressif Blog: “Why We Switched to RISC-V for ESP32-C3”: A great article explaining the company’s strategy and the benefits of the new architecture. (A web search for this title will find it).
• RISC-V International: The official organization for the RISC-V standard.
  • RISC-V International Website