Optional Settings

There are a number of low-level optional settings that can be configured. These are supported by the various XJTAG applications at runtime, but cannot be configured via the user interface; they can only be set by modifying the project file directly.

Disable IDCODE check

In XJEase, at the point before the first JTAG access is performed, XJTAG will first scan the ID codes of the devices in the chain and verify that the expected devices are present in the chain. There are some circumstances where the ID codes cannot be read correctly, in which case this functionality can be disabled by adding this line to the JTAG chain description:

  IDCODE_CHECK := FALSE;

Disable Autoskew

When a specific TCK frequency is used for the first time, XJTAG tries to automatically calculate how much skew must be applied to the JTAG pins to account for delays in the JTAG signals. There are certain situations where this does not work correctly or reliably. This process can be disabled by adding this line to the JTAG chain description:

  AUTOSKEW := FALSE;

However, this will most likely severely restrict the TCK frequencies that can be used reliably.

Override skew values

Sometimes Autoskew cannot work reliably, but a higher TCK frequency is required. In this case, TDO skew values can be manually configured per TCK frequency and per subchain. There is a block per subchain and then a list of frequencies and associated skew values. If the project is not using dynamic chains, then the normal whole chain can be specified with DEFAULT.

Examples

Override skew values for various frequencies in a project that does not use dynamic chains:

  AUTOSKEW
    SUBCHAIN DEFAULT
      500K := 0x35;
      1M := 0x4f;
      2M := 0x53;
    END;
  END;

Override skew values for a TCK frequency in a project that does uses dynamic chains and has two subchains:

  AUTOSKEW
    SUBCHAIN "chain1"
      10M := 0x4f;
    END;
    SUBCHAIN "chain2"
      10M := 0x105;
    END;
  END;