The ABMs are the heart of the 1149.4 architecture and should be present at every analog pin, as shown in figure 1 (digital pins could have ABMs as well, if the additional test overhead could be tolerated).
Figure 1: The basic 1149.4 architecture.
Test signal application and response capturing through the ABMs takes place by combining serial access to the BS register and analog stimulae access to the ATAP. Similarly to the TBIC, the ABMs comprise a switching structure and a control structure to define the flow of analog signals into and out of the function pins. The switching structure of each ABM is represented in figure 2.
ABMs - The Switching Structure
Figure 2 represents the switching structure of each ABM.
Figure 2: Switching structure of the ABMs.
The switching structure shown comprises one comparator and six switches, which together provide the following functions:
ABMs - The Control Structure
The control structure of each ABM may be represented as shown in figure 3.
Figure 3: Control structure of the ABMs.
The control structure for the ABMs is similar to the same structure in the TBIC and may be summarised as follows:
The four bits in the update stage, together with the instruction decoder outputs MODE1 and MODE2, define the operating mode of the ABM switching structure.