分析原因：

USB 线质量问题或者数据线过长；

电脑主板 USB 供电不足；

操作系统 USB 驱动不完整或者不稳定；

电脑 USB 端口故障；

打印机 USB 端口故障。

解决方案如下：

一：检查打印机是否开机

如果打印机处于关机状态，连接USB设备会出现无法识别的现象，需要开机状态下连接USB。

二：检查电脑和打印机端口问题

请尝试更换电脑后置的其他USB端口。或者换其他电脑的USB端口测试。

因为当电脑或者打印机的USB端口存在硬件的问题，有可能电脑设备管理器里面会出现“Unknown Device”错误，如果换任何的电脑都出现“Unknown Device”有可能是一体机USB端口或者USB数据线存在问题。建议更换一条USB数据线连接尝试。

三：停用USB增强控制器

1.请先断开打印机和电脑连接的USB数据线

2.右键点击【我的电脑】图标，（如：WIN7系统是计算机图标），选择【管理】，找到【设备管理器】-【通用串行总线控制器】-找到英文里面带“Enhanced Host Controller”的名称，根据电脑端口不同，可能显示“Enhanced Host Controller”有的比较多，右键单击“Enhanced Host Controller”选择【停用】。停用以后，重新使用USB连接打印机和电脑，正常情况下能正确识别了。

出现* JLink Info: Could not measure total IR len. TDO is constant high.

很有可能的原因是。。。。你用的是4限制SWD接口，但是在keil的工程设置里忘记把jtag选回SWD，或者说你用的是cortex M0的片子，根本就不支持jtag

在keil的工程设置，jlink设置里把调试接口改成SWD就行了

我也是这个问题 我的是 下载的 钥匙不能用 你给我发个 那 个密钥 [email protected] 我说仿真 Proteus 7 Professional Licence Manager .lnk给我发一个

There are several reasons why the CPU core can not be halted.

Either the memory wait signal of the core is still enabled or CPU disables the clock (wrong PLL clock settings/CPU enters power-save mode).

Therefore the core cannot communicate via JTAG.

The only way to gain control of the core via JTAG is to reset it and halt it immediately.

BTW: Which STR7xx are you using?

Could you please do the following:

Start the J-Link commander (JLink.exe)

Set a very high JTAG-speed by typing "speed [speed in kHz], for example "speed 8000" and press enter. If you are using a STR75x core - a TDMI-S core, this may not work.

Now type "rx 0" - This will reset the core as quickly as possible. The higher the speed the earlier the core is stopped.

You should see:

Reset delay: 0 ms

Reset type NORMAL: Using RESET pin, halting CPU after Reset

Info: Resetting target using RESET pin

The core is halted after reset. Type "regs" to see the CPU registers:

PC: (R15) = 00000000, CPSR = 000000D3 (SVC mode, ARM FIQ dis. IRQ dis.)

R0 = A0000054, R1 = 00008040, R2 = 00000000, R3 = 20000004

R4 = 000000E1, R5 = 000000C7, R6 = 0000004D, R7 = 000000BD

USR: R8 =00000000, R9 =00000000, R10=00000001, R11 =00000001, R12 =00000099

R13=200007FC, R14=000000C7

FIQ: R8 =00000000, R9 =00000000, R10=00000000, R11 =00000000, R12 =00000000

R13=04000000, R14=00000000, SPSR=C00000FF

SVC: R13=00000000, R14=000000F4, SPSR=000000FF

ABT: R13=00000000, R14=00000000, SPSR=D000005F

IRQ: R13=20000900, R14=00000000, SPSR=F00000F7

UND: R13=00000000, R14=00000000, SPSR=D000007A

If you can see the registers' contents, the next step will be to erase the contents of the flash. Use J-Flash and set the following settings:

JTAG\JTAG speed before/after init to 8000kHz (or the value you tried with J-Link commander)

CPU\Init sequence: Action type: Reset, Type: 0, Delay: 0ms

Now connect to the target and select "Erase chip" to erase the flash contents.

卸载segger，下载了比较新的驱动重新安装，并将segger/jlinkarm_v440目录下的jlinkarm.dll拷贝到keil/arm/segger的目录下。再次使用MDK，提示升级，确定升级，但这次升级后，jlink确实可以识别。

那就可能是电路的问题了，诡异的问题再开发中是很常见的，可以的话就行了！！