原创 RESERVEDPIN原来并不是都可以悬空的

JTAG调试老是不稳定，刚开始怀疑是芯片焊接的问题。可是又焊了两块，结果还是一样。

又细细看了一下DATASHEET，吓了我一跳。6205上共有12个RESERVED PIN，其中9个标明需要悬空，另外有3个RSV0，1，2是要上拉的。今天直接在TI网站上写技术支持要求。

[Problem:
TMS320C6205 has 12 reserved pins rsv0~rsv11. refer to the datasheet rsv0,1,2 should pullup by 20K resistor and others should left open. I design a board with all reserved pins not connected. When I debug the board with a seed jtag it seems that boot configuration can't read properly. Do reserved pins correlate to boot config settings?]
答复如下：

The reserved pins were used by our factory to test the DSP functionality. If you did not follows the instruction to terminate the pins, we can't guarantee the working of the DSP.

Solution:
Regarding to your question, as you know from the data sheet that these pins are reserved pins for testing. These pins are used to place the device in a test mode and you MUST pull these pins up with resistors for correct operation. It is recommended to follow the data sheet specification. If you are going for large scale production I would recommend you to redesign the PCB to pull-up all the three pins. Otherwise if it is a prototype, I would suggest to pull-up those pins in your existing PCB by doing some modification (if possible), like connecting a pin (RSV0/1/2) to nearest supply voltage line through a resistor. Undesired operation may occur if these pins are left floating. Excessive heating, bus contention, and incorrect clocking are a few examples of what can happen if these pins are not pulled according to the data sheet.

焊盘藏在芯片下了，没法改了。六层板，好几千圆啊！我都要哭了！

只好重新做板了，以后凡是BGA封装的，空引脚也要用过孔或连线连到底层或顶层。至少弄错了还能飞线。