The standard defines 10 switching
patterns for the TBIC (among the 1.024 possible combinations), where each
pattern represents a valid combination of switch conditions. These 10
patterns are presented in table 1 below, together with a short description
of the corresponding operating modes.
The main testing conditions are represented by switching patterns P1 to P3, which connect at least one of the ATAP pins to the internal analog test bus lines (P1 or P2 will clamp the ATAP pin not connected to the internal test bus). Figure 1 shows the effect of switching pattern P1, meant to support observability-only operations.
The selection of each switching pattern is a function of the 4-bit code word shifted into the TBIC control structure (to be presented shortly ahead) and of the current instruction, as described in table 2.
(an asterisk indicates a code word / instruction combination for which the current version of the standard does not define any specific operating mode)
Notice that the same 4-bit code word may lead to different operating modes, according to the contents of the instruction register. Figure 2 shows the operating mode defined for the TBIC switching structure, when the 4-bit code word is 0101 and the current instruction is EXTEST, CLAMP or BYPASS. However, if the current instruction is PROBE or INTEST, the operating mode of the switching structure is not defined by the current version of the standard.