• U2 STM32L152RET6: STM32 HAL / STM32CubeIDE or PlatformIO; primary ADS1299 acquisition controller.
• U3 ESP32-WROOM-32E: ESP32 Arduino or ESP-IDF; wireless bridge and microSD logger.

C
#include "main.h"
#include <stdint.h>

SPI_HandleTypeDef hspi1; // ADS1299
SPI_HandleTypeDef hspi2; // ESP32 bridge

#define ADS_CS_GPIO_Port GPIOB
#define ADS_CS_Pin GPIO_PIN_6
#define ADS_RESET_GPIO_Port GPIOC
#define ADS_RESET_Pin GPIO_PIN_7
#define ADS_DRDY_GPIO_Port GPIOA
#define ADS_DRDY_Pin GPIO_PIN_9

static void ADS1299_Select(void) { HAL_GPIO_WritePin(ADS_CS_GPIO_Port, ADS_CS_Pin, GPIO_PIN_RESET); }
static void ADS1299_Deselect(void) { HAL_GPIO_WritePin(ADS_CS_GPIO_Port, ADS_CS_Pin, GPIO_PIN_SET); }

static HAL_StatusTypeDef ADS1299_Command(uint8_t cmd) {
    ADS1299_Select();
    HAL_StatusTypeDef st = HAL_SPI_Transmit(&hspi1, &cmd, 1, 100);
    ADS1299_Deselect();
    return st;
}

int main(void) {
    HAL_Init();
    SystemClock_Config();
    MX_GPIO_Init();
    MX_SPI1_Init();
    MX_SPI2_Init();

    HAL_GPIO_WritePin(ADS_CS_GPIO_Port, ADS_CS_Pin, GPIO_PIN_SET);
    HAL_GPIO_WritePin(ADS_RESET_GPIO_Port, ADS_RESET_Pin, GPIO_PIN_RESET);
    HAL_Delay(10);
    HAL_GPIO_WritePin(ADS_RESET_GPIO_Port, ADS_RESET_Pin, GPIO_PIN_SET);
    HAL_Delay(100);

    ADS1299_Command(0x11); // SDATAC before register configuration

    while (1) {
        if (HAL_GPIO_ReadPin(ADS_DRDY_GPIO_Port, ADS_DRDY_Pin) == GPIO_PIN_RESET) {
            uint8_t tx[27] = {0};
            uint8_t rx[27] = {0};
            ADS1299_Select();
            HAL_SPI_TransmitReceive(&hspi1, tx, rx, sizeof(rx), 100);
            ADS1299_Deselect();
            HAL_SPI_Transmit(&hspi2, rx, sizeof(rx), 100);
        }
    }
}

Cpp
#include <Arduino.h>
#include <SPI.h>
#include <SD.h>
#include <WiFi.h>

#define STM32_SPI_SCK   18
#define STM32_SPI_MISO  19
#define STM32_SPI_MOSI  23
#define STM32_SPI_CS    5
#define STM32_READY     27

#define SD_CLK          14
#define SD_MISO         12
#define SD_MOSI         13
#define SD_CS           15

SPIClass sdSPI(VSPI);

void setup() {
  Serial.begin(115200);
  pinMode(STM32_READY, INPUT);
  pinMode(STM32_SPI_CS, INPUT_PULLUP);

  sdSPI.begin(SD_CLK, SD_MISO, SD_MOSI, SD_CS);
  if (!SD.begin(SD_CS, sdSPI)) {
    Serial.println("microSD init failed");
  } else {
    Serial.println("microSD ready");
  }

  WiFi.mode(WIFI_STA);
  Serial.println("ESP32 EEG bridge ready");
}

void loop() {
  // TODO: implement STM32 SPI slave/handshake protocol or use an ESP32-supported slave library.
  delay(1000);
}

• Use SWD header J1 for STM32 programming.
• Use USB-C/CP2102N and EN/IO0 buttons or auto-reset for ESP32 programming.
• Confirm IO12 remains low at ESP32 boot because it is both microSD MISO and a boot strap pin.