用直接寄存器KEY0- KEY3分别接PB0-PB3口，LED数码管接在PC0-PC6口。按下KEY0，LED数码管显示0；按下KEY1，LED数码管显示1；按下KEY2，LED数码管显示2；按下KEY3，LED数码管显示3；KEY弹起，LED数码管熄灭。

这个任务可以用STM32单片机来实现。首先，需要配置GPIO口，将PB0-PB3和PC0-PC6设置为输入和输出口。然后，需要设置外部中断，以便在按下和松开按键时产生中断。在中断处理程序中，可以根据按下的按键，设置LED数码管的显示值。以下是一个简单的实现示例：

#include "stm32f10x.h"

void GPIO_Configuration(void);
void EXTI_Configuration(void);
void NVIC_Configuration(void);
void Delay(__IO uint32_t nCount);

int main(void)
{
  GPIO_Configuration();
  EXTI_Configuration();
  NVIC_Configuration();

  while (1)
  {
    // 等待按键中断
  }
}

void GPIO_Configuration(void)
{
  GPIO_InitTypeDef GPIO_InitStructure;

  RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB | RCC_APB2Periph_GPIOC, ENABLE);

  // 配置PB0-PB3为输入口
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0 | GPIO_Pin_1 | GPIO_Pin_2 | GPIO_Pin_3;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IPU;
  GPIO_Init(GPIOB, &GPIO_InitStructure);

  // 配置PC0-PC6为输出口
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0 | GPIO_Pin_1 | GPIO_Pin_2 | GPIO_Pin_3 | GPIO_Pin_4 | GPIO_Pin_5 | GPIO_Pin_6;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
  GPIO_Init(GPIOC, &GPIO_InitStructure);
}

void EXTI_Configuration(void)
{
  EXTI_InitTypeDef EXTI_InitStructure;

  // 配置PB0-PB3为外部中断输入口
  GPIO_EXTILineConfig(GPIO_PortSourceGPIOB, GPIO_PinSource0);
  GPIO_EXTILineConfig(GPIO_PortSourceGPIOB, GPIO_PinSource1);
  GPIO_EXTILineConfig(GPIO_PortSourceGPIOB, GPIO_PinSource2);
  GPIO_EXTILineConfig(GPIO_PortSourceGPIOB, GPIO_PinSource3);

  // 配置外部中断
  EXTI_InitStructure.EXTI_Line = EXTI_Line0 | EXTI_Line1 | EXTI_Line2 | EXTI_Line3;
  EXTI_InitStructure.EXTI_Mode = EXTI_Mode_Interrupt;
  EXTI_InitStructure.EXTI_Trigger = EXTI_Trigger_Rising_Falling;
  EXTI_InitStructure.EXTI_LineCmd = ENABLE;
  EXTI_Init(&EXTI_InitStructure);
}

void NVIC_Configuration(void)
{
  NVIC_InitTypeDef NVIC_InitStructure;

  // 配置中断优先级
  NVIC_PriorityGroupConfig(NVIC_PriorityGroup_2);

  NVIC_InitStructure.NVIC_IRQChannel = EXTI0_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);

  NVIC_InitStructure.NVIC_IRQChannel = EXTI1_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);

  NVIC_InitStructure.NVIC_IRQChannel = EXTI2_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 2;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);

  NVIC_InitStructure.NVIC_IRQChannel = EXTI3_IRQn;
  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
  NVIC_InitStructure.NVIC_IRQChannelSubPriority = 3;
  NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
  NVIC_Init(&NVIC_InitStructure);
}

void Delay(__IO uint32_t nCount)
{
  while (nCount--)
  {
  }
}

void EXTI0_IRQHandler(void)
{
  if (GPIO_ReadInputDataBit(GPIOB, GPIO_Pin_0) == RESET)
  {
    GPIO_Write(GPIOC, 0x00);
    GPIO_SetBits(GPIOC, GPIO_Pin_0);
  }
  else
  {
    GPIO_ResetBits(GPIOC, GPIO_Pin_0);
  }

  EXTI_ClearITPendingBit(EXTI_Line0);
}

void EXTI1_IRQHandler(void)
{
  if (GPIO_ReadInputDataBit(GPIOB, GPIO_Pin_1) == RESET)
  {
    GPIO_Write(GPIOC, 0x00);
    GPIO_SetBits(GPIOC, GPIO_Pin_1);
  }
  else
  {
    GPIO_ResetBits(GPIOC, GPIO_Pin_1);
  }

  EXTI_ClearITPendingBit(EXTI_Line1);
}

void EXTI2_IRQHandler(void)
{
  if (GPIO_ReadInputDataBit(GPIOB, GPIO_Pin_2) == RESET)
  {
    GPIO_Write(GPIOC, 0x00);
    GPIO_SetBits(GPIOC, GPIO_Pin_2);
  }
  else
  {
    GPIO_ResetBits(GPIOC, GPIO_Pin_2);
  }

  EXTI_ClearITPendingBit(EXTI_Line2);
}

void EXTI3_IRQHandler(void)
{
  if (GPIO_ReadInputDataBit(GPIOB, GPIO_Pin_3) == RESET)
  {
    GPIO_Write(GPIOC, 0x00);
    GPIO_SetBits(GPIOC, GPIO_Pin_3);
  }
  else
  {
    GPIO_ResetBits(GPIOC, GPIO_Pin_3);
  }

  EXTI_ClearITPendingBit(EXTI_Line3);
}

在上面的代码中，我们使用了外部中断和中断处理程序来检测按键的状态，并根据按键状态设置LED数码管的显示值。注意，在中断处理程序中，需要清除中断标志位。另外，为了让LED数码管的显示更加稳定，我们在延时函数中添加了一些空循环。