1.2.6. Accessing long scan chains

TAPOp_AccessDR_W calls contain WRoffset and WRmask parameters. In addition, TAPOp_AccessDR_RW calls contain an RDoffset parameter. The purpose of these parameters is to reduce the number of bits that get written to less than 40 bits using WRmask, or increase the number of bits that can be read/written to more than 40 bits by supplying a read/write offset (RDoffset and WRoffset parameters). For example, to access a 50-bit scan chain use the following steps:

  1. Access 40 bits using WRmask = all ones.


    Because the WRmask parameter is a pointer to an array of bits, you can use a shortcut, specifying NULL pointer to mean a mask that is all ones. This only works for a direct TAPOp call, not a macro call.

  2. Access the remaining 10 bits using a 10-bit WRmask and a 40-bit offset.


To access a scan chain over 255 bits in length, you must use the TAPOp_AnySequence_RW and TAPOp_AnySequence_W calls to navigate the JTAG state machine and to perform the read or write of the scan chain in the Shift-DR state.

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