如果ADC转换由定时器触发不起作用，可能是因为以下原因：

1. 定时器配置错误：请确保定时器已正确配置，并且已启动。

2. ADC配置错误：请确保ADC通道和采样时间已正确配置，并且已启动。

以下是一个使用定时器触发ADC转换的示例代码（使用STM32Cube HAL库）：

#include "stm32l4xx.h"
#include "stm32l4xx_hal.h"

ADC_HandleTypeDef hadc1;
TIM_HandleTypeDef htim2;

void SystemClock_Config(void);
void Error_Handler(void);
static void MX_GPIO_Init(void);
static void MX_ADC1_Init(void);
static void MX_TIM2_Init(void);

int main(void)
{
  HAL_Init();
  SystemClock_Config();
  
  MX_GPIO_Init();
  MX_ADC1_Init();
  MX_TIM2_Init();
  
  HAL_TIM_Base_Start(&htim2);  // 启动定时器
  
  while (1)
  {
    HAL_ADC_Start(&hadc1);  // 启动ADC转换
    
    HAL_ADC_PollForConversion(&hadc1, HAL_MAX_DELAY);  // 等待转换完成
    
    uint32_t adc_value = HAL_ADC_GetValue(&hadc1);  // 获取转换结果
    
    HAL_ADC_Stop(&hadc1);  // 停止ADC转换
  }
}

void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct;
  RCC_ClkInitTypeDef RCC_ClkInitStruct;

  __HAL_RCC_PWR_CLK_ENABLE();
  __HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE1);

  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
  RCC_OscInitStruct.HSIState = RCC_HSI_ON;
  RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    Error_Handler();
  }

  RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
                              |RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;
  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_HSI;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_0) != HAL_OK)
  {
    Error_Handler();
  }

  HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000);
  HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);

  HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0);
}

static void MX_GPIO_Init(void)
{
  GPIO_InitTypeDef GPIO_InitStruct;

  __HAL_RCC_GPIOA_CLK_ENABLE();

  GPIO_InitStruct.Pin = GPIO_PIN_0;
  GPIO_InitStruct.Mode = GPIO_MODE_ANALOG;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);
}

static void MX_ADC1_Init(void)
{
  ADC_ChannelConfTypeDef sConfig;

  __HAL_RCC_ADC_CLK_ENABLE();

  hadc1.Instance = ADC1;
  hadc1.Init.ClockPrescaler = ADC_CLOCK_SYNC_PCLK_DIV4;
  hadc1.Init.Resolution = ADC_RESOLUTION_12B;
  hadc1.Init.DataAlign = ADC_DATAALIGN_RIGHT;
  hadc1.Init.ScanConvMode = ADC_SCAN_DISABLE;
  hadc1.Init.EOCSelection = ADC_EOC_SINGLE_CONV;
  hadc1.Init.LowPowerAutoWait = DISABLE;
  hadc1.Init.LowPowerAutoPowerOff = DISABLE;
  hadc1.Init.ChannelsBank = ADC_CHANNELS_BANK_A;
  hadc1.Init.ContinuousConvMode = DISABLE;
  hadc1.Init.NbrOfConversion = 1;
  hadc1.Init.DiscontinuousConvMode = DISABLE;
  hadc1.Init.NbrOfDiscConversion = 0;
  hadc1.Init.ExternalTrigConv = ADC_EXTERNALTRIG_T2_TRGO;  // 选择定时器2触发
  hadc1.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_RISING;  // 上升沿触发
  hadc1.Init.DMAContinuousRequests = DISABLE;
  hadc1.Init.Overrun = ADC_OVR_DATA_OVERWRITTEN;
  if (HAL