背景
很多朋友因爲瞭解到樹莓派的強大，所以都很快入手了一個準備把玩，可是通常買回後，發現自己沒有HDMI顯示器，即使有一個顯示器，也通常是VGA接口的，缺個VGA轉HDMI接頭。其實，沒有顯示器也還有辦法可以通過一臺主機(安裝有ssh或串口通信的軟件)的命令行遠程登錄到樹莓派，通常有兩種方式：
我們主要講解通過串口登錄，因爲這方面的資料介紹比較少。實際中又通常有很多人要通過串口登錄。因爲他們更加令人受不了的是，自己居然沒有路由器 -_-#, 即使有，也不會尋找DHCP分配給派的ip地址，也就無法通過puTTY這類軟件遠程登錄了。
進入正題
要進行串口通信，首先要有一條USB-TTL轉換線和三根杜邦線，杜邦線用來連接USB串口轉換器和樹莓派GPIO的UART針腳（6、8、10號）。連接方式：
| raspberry pi GPIO pin | USB-TTL Pin |
|---|---|
| 6 | GND |
| 8 | RXD |
| 10 | TXD |
首次給樹莓派燒完官方系統後是無法通過串口登錄的，因爲官方默認沒有開啓串口用來輸出啓動信息。
開啓UART串口調試
只需要在燒完官方系統的TF卡上找到config.txt文件，然後再最後面添加一句：
enable_uart=1保存後，首先打開自己PC端的串口通信軟件（如puTTY），設置串口波特率爲115200，這是默認的串口通信波特率與樹莓派的串口波特率一致才能正確通信。 之後插卡啓動樹莓派，就會在終端窗口看到啓動信息了。
[ 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: