static void
low_level_init(struct netif *netif)
{
  struct ethernetif *ethernetif = netif->state;
 
  /* set MAC hardware address length */
  netif->hwaddr_len = ETHARP_HWADDR_LEN;

  /* set MAC hardware address */
  netif->hwaddr[0] = 'A';
  netif->hwaddr[1] = 'R';
  netif->hwaddr[2] = 'M';
  netif->hwaddr[3] = 'N';
  netif->hwaddr[4] = 'E';
  netif->hwaddr[5] = 'T';
 
  /* maximum transfer unit */
  netif->mtu = 1500;
 
  /* device capabilities */
  /* don't set NETIF_FLAG_ETHARP if this device is not an ethernet one */
  netif->flags = NETIF_FLAG_BROADCAST | NETIF_FLAG_ETHARP | NETIF_FLAG_LINK_UP;
 
  /* Do whatever else is needed to initialize interface. */
  enc28j60_init(netif->hwaddr); // 【1】
}

static err_t
low_level_output(struct netif *netif, struct pbuf *p)
{
  struct ethernetif *ethernetif = netif->state;
  struct pbuf *q;

  enc28j60_init_send(p->tot_len); //【1】initiate transfer();
 
#if ETH_PAD_SIZE
  pbuf_header(p, -ETH_PAD_SIZE); /* drop the padding word */
#endif

  for(q = p; q != NULL; q = q->next) {
    /* Send the data from the pbuf to the interface, one pbuf at a
       time. The size of the data in each pbuf is kept in the ->len
       variable. */
    enc28j60_writebuf( q->payload, q->len ); //【2】send data from(q->payload, q->len);
  }

  enc28j60_start_send(); //【3】signal that packet should be sent();

#if ETH_PAD_SIZE
  pbuf_header(p, ETH_PAD_SIZE); /* reclaim the padding word */
#endif
 
  LINK_STATS_INC(link.xmit);

  return ERR_OK;
}

static struct pbuf *
low_level_input(struct netif *netif)
{
  struct ethernetif *ethernetif = netif->state;
  struct pbuf *p, *q;
  u16_t len;

  /* Obtain the size of the packet and put it into the "len"
     variable. */
  len = enc28j60_packet_getlen(); // 【1】

#if ETH_PAD_SIZE
  len += ETH_PAD_SIZE; /* allow room for Ethernet padding */
#endif

  /* We allocate a pbuf chain of pbufs from the pool. */
  p = pbuf_alloc(PBUF_RAW, len, PBUF_POOL);
 
  if (p != NULL) {

#if ETH_PAD_SIZE
    pbuf_header(p, -ETH_PAD_SIZE); /* drop the padding word */
#endif

    /* We iterate over the pbuf chain until we have read the entire
     * packet into the pbuf. */
    for(q = p; q != NULL; q = q->next) {
      /* Read enough bytes to fill this pbuf in the chain. The
       * available data in the pbuf is given by the q->len
       * variable.
       * This does not necessarily have to be a memcpy, you can also preallocate
       * pbufs for a DMA-enabled MAC and after receiving truncate it to the
       * actually received size. In this case, ensure the tot_len member of the
       * pbuf is the sum of the chained pbuf len members.
       */
      enc28j60_readbuf (q->payload, q->len ); //【2】read data into(q->payload, q->len);
    }
    enc28j60_finish_receive(); //【3】acknowledge that packet has been read();

#if ETH_PAD_SIZE
    pbuf_header(p, ETH_PAD_SIZE); /* reclaim the padding word */
#endif

    LINK_STATS_INC(link.recv);
  } else {
    enc28j60_finish_receive(); //【4】drop packet();
    LINK_STATS_INC(link.memerr);
    LINK_STATS_INC(link.drop);
  }

  return p;
}

#ifndef __LWIPOPTS_H__
#define __LWIPOPTS_H__

/**
 * SYS_LIGHTWEIGHT_PROT==1: if you want inter-task protection for certain
 * critical regions during buffer allocation, deallocation and memory
 * allocation and deallocation.
 */
#define SYS_LIGHTWEIGHT_PROT 0

/**
 * NO_SYS==1: Provides VERY minimal functionality. Otherwise,
 * use lwIP facilities.
 */
#define NO_SYS 1
/**
 * NO_SYS_NO_TIMERS==1: Drop support for sys_timeout when NO_SYS==1
 * Mainly for compatibility to old versions.
 */
#define NO_SYS_NO_TIMERS 1

/* ---------- Memory options ---------- */
/* MEM_ALIGNMENT: should be set to the alignment of the CPU for which
   lwIP is compiled. 4 byte alignment -> define MEM_ALIGNMENT to 4, 2
   byte alignment -> define MEM_ALIGNMENT to 2. */
#define MEM_ALIGNMENT 4

/* MEM_SIZE: the size of the heap memory. If the application will send
a lot of data that needs to be copied, this should be set high. */
#define MEM_SIZE (5*1024)

/* MEMP_NUM_PBUF: the number of memp struct pbufs. If the application
   sends a lot of data out of ROM (or other static memory), this
   should be set high. */
#define MEMP_NUM_PBUF 10
/* MEMP_NUM_UDP_PCB: the number of UDP protocol control blocks. One
   per active UDP "connection". */
#define MEMP_NUM_UDP_PCB 6
/* MEMP_NUM_TCP_PCB: the number of simulatenously active TCP
   connections. */
#define MEMP_NUM_TCP_PCB 10
/* MEMP_NUM_TCP_PCB_LISTEN: the number of listening TCP
   connections. */
#define MEMP_NUM_TCP_PCB_LISTEN 6
/* MEMP_NUM_TCP_SEG: the number of simultaneously queued TCP
   segments. */
#define MEMP_NUM_TCP_SEG 12
/* MEMP_NUM_SYS_TIMEOUT: the number of simulateously active
   timeouts. */
#define MEMP_NUM_SYS_TIMEOUT 3


/* ---------- Pbuf options ---------- */
/* PBUF_POOL_SIZE: the number of buffers in the pbuf pool. */
#define PBUF_POOL_SIZE 10

/* PBUF_POOL_BUFSIZE: the size of each pbuf in the pbuf pool. */
#define PBUF_POOL_BUFSIZE 1500


/* ---------- TCP options ---------- */
#define LWIP_TCP 1
#define TCP_TTL 255

/* Controls if TCP should queue segments that arrive out of
   order. Define to 0 if your device is low on memory. */
#define TCP_QUEUE_OOSEQ 0

/* TCP Maximum segment size. */
#define TCP_MSS (1500 - 40)	 /* TCP_MSS = (Ethernet MTU - IP header size - TCP header size) */

/* TCP sender buffer space (bytes). */
#define TCP_SND_BUF (2*TCP_MSS)

/* TCP sender buffer space (pbufs). This must be at least = 2 *
   TCP_SND_BUF/TCP_MSS for things to work. */
#define TCP_SND_QUEUELEN (6 * TCP_SND_BUF)/TCP_MSS

/* TCP receive window. */
#define TCP_WND (2*TCP_MSS)


/* ---------- ICMP options ---------- */
#define LWIP_ICMP 1


/* ---------- DHCP options ---------- */
/* Define LWIP_DHCP to 1 if you want DHCP configuration of
   interfaces. DHCP is not implemented in lwIP 0.5.1, however, so
   turning this on does currently not work. */
#define LWIP_DHCP 0


/* ---------- UDP options ---------- */
#define LWIP_UDP 1
#define UDP_TTL 255


/* ---------- Statistics options ---------- */
#define LWIP_STATS 0
#define LWIP_PROVIDE_ERRNO 1


/*
   --------------------------------------
   ---------- Checksum options ----------
   --------------------------------------
*/


/*
   ----------------------------------------------
   ---------- Sequential layer options ----------
   ----------------------------------------------
*/
/**
 * LWIP_NETCONN==1: Enable Netconn API (require to use api_lib.c)
 */
#define LWIP_NETCONN 0

/*
   ------------------------------------
   ---------- Socket options ----------
   ------------------------------------
*/
/**
 * LWIP_SOCKET==1: Enable Socket API (require to use sockets.c)
 */
#define LWIP_SOCKET 0

#endif /* __LWIPOPTS_H__ */

void LwIP_Config (void)
{
    struct ip_addr ipaddr;
    struct ip_addr netmask;
    struct ip_addr gw;
   
    // 调用LWIP初始化函数
    lwip_init();
   
    IP4_ADDR(&ipaddr, 192, 168, 1, 16); // 设置网络接口的ip地址
    IP4_ADDR(&netmask, 255, 255, 255, 0);	 // 子网掩码
    IP4_ADDR(&gw, 192, 168, 1, 1);	 // 网关
   
    // 初始化enc28j60与LWIP的接口，参数为网络接口结构体、ip地址、
    // 子网掩码、网关、网卡信息指针、初始化函数、输入函数
    netif_add(&enc28j60, &ipaddr, &netmask, &gw, NULL, ðernetif_init, ðernet_input);
   
    // 把enc28j60设置为默认网卡
    netif_set_default(&enc28j60);
   
    netif_set_up(&enc28j60);
}

    timer_typedef tcp_timer, arp_timer;
   
    /* 设定查询定时器 ARP定时器 */
    timer_set(&tcp_timer, CLOCK_SECOND / 10); // tcp处理定时器 100ms
    timer_set(&arp_timer, CLOCK_SECOND * 5); // arp处理定时器 5s
    while (1) {
       
        if (enc28j60_packet_getcount() != 0) {
            ethernetif_input(&enc28j60);
        }
       
        // TCP 定时处理
        if (timer_expired(&tcp_timer)) {
            timer_set(&tcp_timer, CLOCK_SECOND / 4);
            tcp_tmr();
        }
       
        // ARP 定时处理
        if (timer_expired(&arp_timer)) {
            timer_set(&arp_timer, CLOCK_SECOND * 5);
            etharp_tmr();
        }
    }