通过串口UART登录树莓派

背景

很多朋友因为了解到树莓派的强大，所以都很快入手了一个准备把玩，可是通常买回后，发现自己没有HDMI显示器，即使有一个显示器，也通常是VGA接口的，缺个VGA转HDMI接头。其实，没有显示器也还有办法可以通过一台主机(安装有ssh或串口通信的软件)的命令行远程登录到树莓派，通常有两种方式：

• 通过网络连接，使用ssh远程登录(这种方式官网文档里说的很详细)
• 通过串口远程登录

我们主要讲解通过串口登录，因为这方面的资料介绍比较少。实际中又通常有很多人要通过串口登录。因为他们更加令人受不了的是，自己居然没有路由器 -_-#, 即使有，也不会寻找DHCP分配给派的ip地址，也就无法通过puTTY这类软件远程登录了。

进入正题

要进行串口通信，首先要有一条USB-TTL转换线和三根杜邦线，杜邦线用来连接USB串口转换器和树莓派GPIO的UART针脚（6、8、10号）。连接方式：

raspberry pi GPIO pin USB-TTL Pin 6 GND 8 RXD 10 TXD

首次给树莓派烧完官方系统后是无法通过串口登录的，因为官方默认没有开启串口用来输出启动信息。

开启UART串口调试

只需要在烧完官方系统的TF卡上找到config.txt文件，然后再最后面添加一句：

enable_uart=1

保存后，首先打开自己PC端的串口通信软件（如puTTY），设置串口波特率为115200，这是默认的串口通信波特率与树莓派的串口波特率一致才能正确通信。 之后插卡启动树莓派，就会在终端窗口看到启动信息了。

[    0.000000] Booting Linux on physical CPU 0x0[    0.000000] Initializing cgroup subsys cpuset[    0.000000] Initializing cgroup subsys cpu[    0.000000] Initializing cgroup subsys cpuacct[    0.000000] Linux version 4.4.9-v7+ (dc4@dc4-XPS13-9333) (gcc version 4.9.3 6[    0.000000] CPU: ARMv7 Processor [410fd034] revision 4 (ARMv7), cr=10c5383d[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT aliasing instructie[    0.000000] Machine model: Raspberry Pi 3 Model B Rev 1.2[    0.000000] cma: Reserved 8 MiB at 0x3a800000[    0.000000] Memory policy: Data cache writealloc[    0.000000] [bcm2709_smp_init_cpus] enter (9520->f3003010)[    0.000000] [bcm2709_smp_init_cpus] ncores=4[    0.000000] PERCPU: Embedded 13 pages/cpu @b9f63000 s22592 r8192 d22464 u5328[    0.000000] Built 1 zonelists in Zone order, mobility grouping on.  Total pa0[    0.000000] Kernel command line: 8250.nr_uarts=1 dma.dmachans=0x7f35 bcm2708t[    0.000000] PID hash table entries: 4096 (order: 2, 16384 bytes)             [    0.000000] Dentry cache hash table entries: 131072 (order: 7, 524288 bytes) [    0.000000] Inode-cache hash table entries: 65536 (order: 6, 262144 bytes)   [    0.000000] Memory: 939088K/966656K available (6340K kernel code, 432K rwdat)[    0.000000] Virtual kernel memory layout:                                    [    0.000000]     vector  : 0xffff0000 - 0xffff1000   (   4 kB)                [    0.000000]     fixmap  : 0xffc00000 - 0xfff00000   (3072 kB)                [    0.000000]     vmalloc : 0xbb800000 - 0xff800000   (1088 MB)                [    0.000000]     lowmem  : 0x80000000 - 0xbb000000   ( 944 MB)                [    0.000000]     modules : 0x7f000000 - 0x80000000   (  16 MB)                [    0.000000]       .text : 0x80008000 - 0x807e5330   (8053 kB)                [    0.000000]       .init : 0x807e6000 - 0x8085e000   ( 480 kB)                [    0.000000]       .data : 0x8085e000 - 0x808ca088   ( 433 kB)                [    0.000000]        .bss : 0x808cd000 - 0x8098c1ac   ( 765 kB)                [    0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=4, Nodes=1       [    0.000000] Hierarchical RCU implementation.                                 [    0.000000]  Build-time adjustment of leaf fanout to 32.                     [    0.000000] NR_IRQS:16 nr_irqs:16 16                                         [    0.000000] Architected cp15 timer(s) running at 19.20MHz (phys).            [    0.000000] clocksource: arch_sys_counter: mask: 0xffffffffffffff max_cycless[    0.000010] sched_clock: 56 bits at 19MHz, resolution 52ns, wraps every 4398s[    0.000027] Switching to timer-based delay loop, resolution 52ns             [    0.000293] Console: colour dummy device 80x30                               [    0.001344] console [tty1] enabled                                           [    0.001398] Calibrating delay loop (skipped), value calculated using timer f)[    0.001466] pid_max: default: 32768 minimum: 301                             [    0.001799] Mount-cache hash table entries: 2048 (order: 1, 8192 bytes)      [    0.001842] Mountpoint-cache hash table entries: 2048 (order: 1, 8192 bytes) [    0.002800] Disabling cpuset control group subsystem                         [    0.002858] Initializing cgroup subsys io                                    [    0.002909] Initializing cgroup subsys memory                                [    0.002972] Initializing cgroup subsys devices                               [    0.003014] Initializing cgroup subsys freezer                               [    0.003056] Initializing cgroup subsys net_cls                               [    0.003128] CPU: Testing write buffer coherency: ok                          [    0.003216] ftrace: allocating 21193 entries in 63 pages                     [    0.052352] CPU0: update cpu_capacity 1024                                   [    0.052416] CPU0: thread -1, cpu 0, socket 0, mpidr 80000000                 [    0.052449] [bcm2709_smp_prepare_cpus] enter                                 [    0.052598] Setting up static identity map for 0x8240 - 0x8274               [    0.054273] [bcm2709_boot_secondary] cpu:1 started (0) 17                    [    0.054600] [bcm2709_secondary_init] enter cpu:1                             [    0.054643] CPU1: update cpu_capacity 1024                                   [    0.054649] CPU1: thread -1, cpu 1, socket 0, mpidr 80000001                 [    0.055024] [bcm2709_boot_secondary] cpu:2 started (0) 16                    [    0.055285] [bcm2709_secondary_init] enter cpu:2                             [    0.055306] CPU2: update cpu_capacity 1024                                   [    0.055312] CPU2: thread -1, cpu 2, socket 0, mpidr 80000002                 [    0.055669] [bcm2709_boot_secondary] cpu:3 started (0) 17                    [    0.055851] [bcm2709_secondary_init] enter cpu:3                             [    0.055871] CPU3: update cpu_capacity 1024                                   [    0.055877] CPU3: thread -1, cpu 3, socket 0, mpidr 80000003                 [    0.055936] Brought up 4 CPUs                                                [    0.056034] SMP: Total of 4 processors activated (153.60 BogoMIPS).          [    0.056064] CPU: All CPU(s) started in HYP mode.                             [    0.056089] CPU: Virtualization extensions available.                        [    0.056728] devtmpfs: initialized                                            [    0.066460] VFP support v0.3: implementor 41 architecture 3 part 40 variant 4[    0.066819] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, ms[    0.067534] pinctrl core: initialized pinctrl subsystem                      [    0.068084] NET: Registered protocol family 16                               [    0.073270] DMA: preallocated 4096 KiB pool for atomic coherent allocations  [    0.080087] bcm2709: Mini UART enabled                                       [    0.080146] hw-breakpoint: found 5 (+1 reserved) breakpoint and 4 watchpoint.[    0.080193] hw-breakpoint: maximum watchpoint size is 8 bytes.               [    0.080370] Serial: AMBA PL011 UART driver                                   [    0.080523] uart-pl011 3f201000.uart: could not find pctldev for node /soc/ge[    0.080725] bcm2835-mbox 3f00b880.mailbox: mailbox enabled                   [    0.143041] bcm2835-dma 3f007000.dma: DMA legacy API manager at f3007000, dm1[    0.144709] SCSI subsystem initialized                                       [    0.144902] usbcore: registered new interface driver usbfs                   [    0.145012] usbcore: registered new interface driver hub                     [    0.145130] usbcore: registered new device driver usb                        [    0.151913] raspberrypi-firmware soc:firmware: Attached to firmware from 2013[    0.179166] clocksource: Switched to clocksource arch_sys_counter            [    0.224478] FS-Cache: Loaded                                                 [    0.224798] CacheFiles: Loaded                                               [    0.236962] NET: Registered protocol family 2                                [    0.237837] TCP established hash table entries: 8192 (order: 3, 32768 bytes) [    0.237975] TCP bind hash table entries: 8192 (order: 4, 65536 bytes)        [    0.238186] TCP: Hash tables configured (established 8192 bind 8192)         [    0.238299] UDP hash table entries: 512 (order: 2, 16384 bytes)              [    0.238367] UDP-Lite hash table entries: 512 (order: 2, 16384 bytes)         [    0.238628] NET: Registered protocol family 1                                [    0.238960] RPC: Registered named UNIX socket transport module.              [    0.238992] RPC: Registered udp transport module.                            [    0.239020] RPC: Registered tcp transport module.                            [    0.239048] RPC: Registered tcp NFSv4.1 backchannel transport module.        [    0.240107] hw perfevents: enabled with armv7_cortex_a7 PMU driver, 7 countee[    0.241544] futex hash table entries: 1024 (order: 4, 65536 bytes)           [    0.254738] VFS: Disk quotas dquot_6.6.0                                     [    0.255058] VFS: Dquot-cache hash table entries: 1024 (order 0, 4096 bytes)  [    0.257470] FS-Cache: Netfs 'nfs' registered for caching                     [    0.258365] NFS: Registering the id_resolver key type                        [    0.258446] Key type id_resolver registered                                  [    0.258475] Key type id_legacy registered                                    [    0.260778] Block layer SCSI generic (bsg) driver version 0.4 loaded (major )[    0.260944] io scheduler noop registered                                     [    0.260982] io scheduler deadline registered (default)                       [    0.261062] io scheduler cfq registered                                      [    0.263572] BCM2708FB: allocated DMA memory fac00000                         [    0.263619] BCM2708FB: allocated DMA channel 0 @ f3007000                    [    0.269285] Console: switching to colour frame buffer device 80x30           [    0.274366] Serial: 8250/16550 driver, 1 ports, IRQ sharing disabled         [    0.276956] console [ttyS0] disabled                                         [    0.278606] 3f215040.uart: ttyS0 at MMIO 0x3f215040 (irq = 59, base_baud = 30[    1.012510] console [ttyS0] enabled                                          [    1.865544] bcm2835-rng 3f104000.rng: hwrng registered                       [    1.872649] vc-cma: Videocore CMA driver                                     [    1.878337] vc-cma: vc_cma_base      = 0x00000000                            [    1.884802] vc-cma: vc_cma_size      = 0x00000000 (0 MiB)                    [    1.891910] vc-cma: vc_cma_initial   = 0x00000000 (0 MiB)                    [    1.899211] vc-mem: phys_addr:0x00000000 mem_base=0x3dc00000 mem_size:0x3f00)[    1.925286] brd: module loaded                                               [    1.938451] loop: module loaded                                              [    1.944029] vchiq: vchiq_init_state: slot_zero = 0xbac80000, is_master = 0   [    1.954020] Loading iSCSI transport class v2.0-870.                          [    1.961191] usbcore: registered new interface driver smsc95xx                [    1.968619] dwc_otg: version 3.00a 10-AUG-2012 (platform bus)                [    2.176252] Core Release: 2.80a                                              [    2.180978] Setting default values for core params                           [    2.187409] Finished setting default values for core params                  [    2.395014] Using Buffer DMA mode                                            [    2.399954] Periodic Transfer Interrupt Enhancement - disabled               [    2.407498] Multiprocessor Interrupt Enhancement - disabled                  [    2.414785] OTG VER PARAM: 0, OTG VER FLAG: 0                                [    2.420818] Dedicated Tx FIFOs mode                                          [    2.426212] WARN::dwc_otg_hcd_init:1047: FIQ DMA bounce buffers: virt = 0xba4[    2.439377] FIQ FSM acceleration enabled for :                               [    2.439377] Non-periodic Split Transactions                                  [    2.439377] Periodic Split Transactions                                      [    2.439377] High-Speed Isochronous Endpoints                                 [    2.439377] Interrupt/Control Split Transaction hack enabled                 [    2.469909] WARN::hcd_init_fiq:413: FIQ on core 1 at 0x804459c4              [    2.477566] WARN::hcd_init_fiq:414: FIQ ASM at 0x80445d34 length 36          [    2.485546] WARN::hcd_init_fiq:439: MPHI regs_base at 0xbb932000             [    2.493272] dwc_otg 3f980000.usb: DWC OTG Controller                         [    2.499937] dwc_otg 3f980000.usb: new USB bus registered, assigned bus numbe1[    2.510586] dwc_otg 3f980000.usb: irq 62, io mem 0x00000000                  [    2.517903] Init: Port Power? op_state=1                                     [    2.523494] Init: Power Port (0)                                             [    2.528515] usb usb1: New USB device found, idVendor=1d6b, idProduct=0002    [    2.537043] usb usb1: New USB device strings: Mfr=3, Product=2, SerialNumber1[    2.547560] usb usb1: Product: DWC OTG Controller                            [    2.553963] usb usb1: Manufacturer: Linux 4.4.9-v7+ dwc_otg_hcd              [    2.561609] usb usb1: SerialNumber: 3f980000.usb                             [    2.568657] hub 1-0:1.0: USB hub found                                       [    2.574085] hub 1-0:1.0: 1 port detected                                     [    2.580519] usbcore: registered new interface driver usb-storage             [    2.588417] mousedev: PS/2 mouse device common for all mice                  [    2.596331] bcm2835-cpufreq: min=600000 max=1200000                          [    2.603109] sdhci: Secure Digital Host Controller Interface driver           [    2.610979] sdhci: Copyright(c) Pierre Ossman                                [    2.617277] sdhost: log_buf @ bac13000 (fac13000)                            [    2.699193] mmc0: sdhost-bcm2835 loaded - DMA enabled (>1)                   [    2.708607] mmc-bcm2835 3f300000.mmc: mmc_debug:0 mmc_debug2:0               [    2.716152] mmc-bcm2835 3f300000.mmc: DMA channel allocated                  [    2.756235] mmc0: host does not support reading read-only switch, assuming we[    2.759278] sdhci-pltfm: SDHCI platform and OF driver helper                 [    2.759675] ledtrig-cpu: registered to indicate activity on CPUs             [    2.759788] hidraw: raw HID events driver (C) Jiri Kosina                    [    2.759981] usbcore: registered new interface driver usbhid                  [    2.759983] usbhid: USB HID core driver                                      [    2.760535] Initializing XFRM netlink socket                                 [    2.760558] NET: Registered protocol family 17                               [    2.760674] Key type dns_resolver registered                                 [    2.761168] Registering SWP/SWPB emulation handler                           [    2.761941] registered taskstats version 1                                   [    2.762136] vc-sm: Videocore shared memory driver                            [    2.762145] [vc_sm_connected_init]: start                                    [    2.767935] [vc_sm_connected_init]: end - returning 0                        [    2.769323] 3f201000.uart: ttyAMA0 at MMIO 0x3f201000 (irq = 87, base_baud =2[    2.769695] of_cfs_init                                                      [    2.769770] of_cfs_init: OK                                                  [    2.779276] Indeed it is in host mode hprt0 = 00021501                       [    2.878496] Waiting for root device /dev/mmcblk0p2...                        [    2.880327] mmc0: new high speed SDHC card at address 0007                   [    2.880931] mmcblk0: mmc0:0007 SD32G 29.0 GiB                                [    2.890352]  mmcblk0: p1 p2                                                  [    2.905435] EXT4-fs (mmcblk0p2): INFO: recovery required on readonly filesysm[    2.909269] mmc1: queuing unknown CIS tuple 0x80 (2 bytes)                   [    2.910816] mmc1: queuing unknown CIS tuple 0x80 (3 bytes)                   [    2.912361] mmc1: queuing unknown CIS tuple 0x80 (3 bytes)                   [    2.915156] mmc1: queuing unknown CIS tuple 0x80 (7 bytes)                   [    2.943856] EXT4-fs (mmcblk0p2): write access will be enabled during recovery[    2.959202] usb 1-1: new high-speed USB device number 2 using dwc_otg        [    2.967349] Indeed it is in host mode hprt0 = 00001101                       [    2.975113] EXT4-fs (mmcblk0p2): recovery complete                           [    3.008351] EXT4-fs (mmcblk0p2): mounted filesystem with ordered data mode. )[    3.019558] VFS: Mounted root (ext4 filesystem) readonly on device 179:2.    [    3.029005] devtmpfs: mounted                                                [    3.034767] Freeing unused kernel memory: 480K (807e6000 - 8085e000)         [    3.045289] mmc1: new high speed SDIO card at address 0001                   [    3.169469] usb 1-1: New USB device found, idVendor=0424, idProduct=9514     [    3.177952] usb 1-1: New USB device strings: Mfr=0, Product=0, SerialNumber=0[    3.187663] hub 1-1:1.0: USB hub found                                       [    3.193275] hub 1-1:1.0: 5 ports detected                                    [    3.311752] random: systemd urandom read with 55 bits of entropy available   [    3.324555] systemd[1]: systemd 215 running in system mode. (+PAM +AUDIT +SE)[    3.341939] systemd[1]: Detected architecture 'arm'.                         [    3.479253] usb 1-1.1: new high-speed USB device number 3 using dwc_otg      [    3.479431] NET: Registered protocol family 10                               [    3.480778] systemd[1]: Inserted module 'ipv6'                               [    3.484035] systemd[1]: Set hostname to <raspberrypi>.                       [    3.619538] usb 1-1.1: New USB device found, idVendor=0424, idProduct=ec00   [    3.628441] usb 1-1.1: New USB device strings: Mfr=0, Product=0, SerialNumbe0[    3.642514] smsc95xx v1.0.4                                                  [    3.713078] smsc95xx 1-1.1:1.0 eth0: register 'smsc95xx' at usb-3f980000.usbf[    3.819225] usb 1-1.5: new full-speed USB device number 4 using dwc_otg      [    3.937391] systemd[1]: Expecting device dev-ttyS0.device...                 [    3.947641] systemd[1]: Starting Forward Password Requests to Wall Directory.[    3.959468] systemd[1]: Started Forward Password Requests to Wall Directory .[    3.971020] systemd[1]: Starting Remote File Systems (Pre).                  [    3.981043] systemd[1]: Reached target Remote File Systems (Pre).            [    3.989317] systemd[1]: Starting Encrypted Volumes.                          [    3.998473] systemd[1]: Reached target Encrypted Volumes.                    [    4.006104] systemd[1]: Starting Arbitrary Executable File Formats File Syst.[    4.021712] systemd[1]: Set up automount Arbitrary Executable File Formats F.[    4.035350] usb 1-1.5: New USB device found, idVendor=248a, idProduct=8566   [    4.035407] systemd[1]: Starting Swap.                                       [    4.050304] usb 1-1.5: New USB device strings: Mfr=1, Product=2, SerialNumbe0[    4.050314] usb 1-1.5: Product: Wireless Receiver                            [    4.050320] usb 1-1.5: Manufacturer: Telink                                  [    4.077599] systemd[1]: Reached target Swap.                                 [    4.084071] systemd[1]: Expecting device dev-mmcblk0p1.device...             [    4.094640] systemd[1]: Starting Root Slice.                                 [    4.103335] systemd[1]: Created slice Root Slice.                            [    4.110175] systemd[1]: Starting User and Session Slice.                     [    4.110607] input: Telink Wireless Receiver as /devices/platform/soc/3f980000[    4.136483] systemd[1]: Created slice User and Session Slice.                [    4.144354] systemd[1]: Starting /dev/initctl Compatibility Named Pipe.      [    4.155413] systemd[1]: Listening on /dev/initctl Compatibility Named Pipe.  [    4.164518] systemd[1]: Starting Delayed Shutdown Socket.                    [    4.172018] hid-generic 0003:248A:8566.0001: input,hidraw0: USB HID v1.11 Ke0[    4.190913] systemd[1]: Listening on Delayed Shutdown Socket.                [    4.198668] systemd[1]: Starting Journal Socket (/dev/log).                  [    4.208574] systemd[1]: Listening on Journal Socket (/dev/log).              [    4.216559] systemd[1]: Starting udev Control Socket.                        [    4.225900] systemd[1]: Listening on udev Control Socket.                    [    4.233386] systemd[1]: Starting udev Kernel Socket.                         [    4.242559] systemd[1]: Listening on udev Kernel Socket.                     [    4.249979] systemd[1]: Starting Journal Socket.                             [    4.258856] systemd[1]: Listening on Journal Socket.                         [    4.265988] systemd[1]: Starting System Slice.                               [    4.274775] systemd[1]: Created slice System Slice.                          [    4.277133] input: Telink Wireless Receiver as /devices/platform/soc/3f980001[    4.298107] systemd[1]: Starting File System Check on Root Device...         [    4.329662] hid-generic 0003:248A:8566.0002: input,hidraw1: USB HID v1.11 Mo1[    4.359781] systemd[1]: Starting system-systemd\x2dfsck.slice.               [    4.370389] systemd[1]: Created slice system-systemd\x2dfsck.slice.          [    4.378809] systemd[1]: Starting system-autologin.slice.                     [    4.389362] systemd[1]: Created slice system-autologin.slice.                [    4.397243] systemd[1]: Starting system-serial\x2dgetty.slice.               [    4.407784] systemd[1]: Created slice system-serial\x2dgetty.slice.          [    4.416334] systemd[1]: Starting Increase datagram queue length...           [    4.429429] systemd[1]: Starting Restore / save the current clock...         [    4.471153] systemd[1]: Starting Load Kernel Modules...                      [    4.483448] systemd[1]: Starting udev Coldplug all Devices...                [    4.497008] systemd[1]: Mounted Huge Pages File System.                      [    4.505295] systemd[1]: Starting Create list of required static device nodes.[    4.572731] systemd[1]: Started Set Up Additional Binary Formats.            [    4.581676] systemd[1]: Mounting POSIX Message Queue File System...          [    4.621927] fuse init (API version 7.23)                                     [    4.660016] systemd[1]: Mounting Debug File System...                        [    4.670488] i2c /dev entries driver                                          [    4.672420] systemd[1]: Starting Slices.                                     [    4.684393] systemd[1]: Reached target Slices.                               [    4.697543] systemd[1]: Started Increase datagram queue length.              [    4.724812] systemd[1]: Started Restore / save the current clock.            [    4.736083] systemd[1]: Started Load Kernel Modules.                         [    4.783579] systemd[1]: Started Create list of required static device nodes .[    4.799779] systemd[1]: Mounted POSIX Message Queue File System.             [    4.810645] systemd[1]: Mounted Debug File System.                           [    4.821645] systemd[1]: Time has been changed                                [    4.831061] systemd[1]: Started udev Coldplug all Devices.                   [    4.962840] systemd[1]: Starting Create Static Device Nodes in /dev...       [    5.009803] systemd[1]: Starting Apply Kernel Variables...                   [    5.021963] systemd[1]: Mounting FUSE Control File System...                 [    5.034783] systemd[1]: Mounting Configuration File System...                [    5.047794] systemd[1]: Starting Syslog Socket.                              [    5.057180] systemd[1]: Listening on Syslog Socket.                          [    5.064278] systemd[1]: Starting Journal Service...                          [    5.078057] systemd[1]: Started Journal Service.                             Raspbian GNU/Linux 8 raspberrypi ttyS0                                          raspberrypi login: 

大功告成