August 14th 2007


  • The smd circuit version is not working as expected. Bluetooth connection cannot be established. The Bluetooth discovery service on PC could not detect the sensing interface's bluetooth. Sometime the connection is made but data received is corrupted. Other function such as sensor data acquisition works fine. In Circuit Serial Programing also run smoothly.
  • After testing with different bluetooth modules, especially those that can run stably on the previous version, there is a chance that the problem comes from the smt PIC. This was verify being true by checking RX/Tx signals from PIC and compare with output signal of the through hole PIC.
  • Further testing with different voltage supplies (5V -> 3.3V) shows that the PIC Tx signals break when supply voltage goes lower than 3.5V. This only occur in the new PCB version but not with the previous PCB ( which use through hole PIC in the same design).

Possible causes:

  • Too many additional wire connection is made in the new PCB to act as via might have affected the PIC performance. (NUS PCB fabrication facility is not able to create vias. Hence wire connection must be make for every via in the design). Verify approach: a new PCB prototype is fabricated. The 4layers design is re-layout to become 2 layers so that the chance of hardware defect is minimize.
  • Additional component for ICSP in the new design might have the side effect on the PIC performance. Verify approach: a SIP PIC is used instead of SOP PIC in the same circuit design. The circuit run smoothly even at 3.3V.

This is a capture of the good TX signal:


Capture of the bad TX signal from new smd circuit PCB

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