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Certain test procedures and production environments require the use of a cable longer than the typical 9-inch flat ribbon cable typically included with P&E hardware interfaces. Extending the cable length requires special considerations for signal integrity, crosstalk, and electromagnetic interference. Simply using a longer cable without understanding these topics will usually produce a setup that does not work reliably, if at all. Extending the ribbon cable should be the option only if you determine that you cannot make longer the length of the USB, Ethernet, or Serial cable that connects the P&E hardware interface to your PC. The cables for the communication ports already have some shielding.

If it is necessary to use a longer ribbon cable, P&E recommends using a shielded jacketed cable . This cable configuration is excellent at reducing crosstalk as well as minimizing electromagnetic interference from other devices. Further improvement can be obtained if the wires are also arranged in twisted pairs.

Shielded USB cables are inexpensive and easy to rework. The four wires provided can be used to create a cable for the standard 6-pin BDM header used by many Freescale microprocessors (such as the HCS08, RS08, CFV1, and HCS12). But this option won't work for other devices that require more debug pins.

In general, these guidelines should be followed for all cables between the target microprocessor and the P&E hardware interface:

  1. Use the shortest cable possible

     

  2. Use shielded cable configurations to reduce parasitic effects

     

  3. Lower the communication frequency. For the ColdFire or Qorivva architectures, the communication frequency is controlled by the BDM Debug Shift Frequency setting. For other architectures, the communication speed is only dependent on the processor's bus frequency. Reducing the bus frequency (ie. disabling the PLL) should improve results.
The debug signals for some ColdFire devices such as the MCF5272 and MCF5206 have to be synchronized before reaching the microprocessor. This additional requirement must be kept in mind.

 





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