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In developing dot4, it was considered important to recognize that the components mounted on a practical mixed-signal PCA would not consist exclusively of those equipped with either dot1 or dot4.

The next figure shows a dot4 component mounted on a typical mixed-signal PCA.

Its pins may be connected to:

* other mixed-signal components (start-view ICs labeled M), which may or may not conform to dot4

* digital components (labeled D), which may or may not conform to dot1

* analog components (labeled A) that are analogous to digital "glue logic." These devices could be anything from a passive analog signal processing circuit to an op-amp or active filter, or even a single transistor. All such elements would be unlikely to contain any associated testability features.

* passive circuit elements (one is labeled E), such as coupling capacitors or pull-up resistors, which will not have any associated testability features.

Dot4 Boundary Modules
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In dot4, every pin is required to have what's called a boundary module associated with it. In the case of the digital pins, the boundary modules (DBMs, or digital boundary modules) are identical to the boundary-scan cells defined in dot1, except that there's an option to extend the facilities using a form of analog boundary module. ABMs are needed if enhanced (analog) measurement capabilities are required.

A standard ABM consists of a switching network connected between the pin and the core circuitry, permitting the pin to be put into the core disconnect (CD) state. In that state it's isolated from the core. The ABM also permits connection to a dc voltage or an internal analog test bus line.

Interconnect testing has always been seen as the most important test capability of dot1. With boundary modules on all pins, giving the ability to apply logic values and to monitor logic responses at all inputs and outputs, all simple interconnect on a dot4 mixed-signal board can be tested using the conventional dot1 protocols.

Extended interconnect can be tested by making analog (parametric) measurements using the analog test buses to apply test stimuli and to monitor analog test responses.

Achieving Control
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Each DBM contains a memory element that can store logic test stimuli or responses. These memory elements appear as part of the boundary-scan register, and are accessed by external test equipment through TDI and TDO under the control of the TAP controller.

The ABMs are also controlled digitally, the operation of the switches in each ABM being determined by the contents of its individual control register. The control register can also be used to capture digital test results, representing either logic values or digitized analog responses. All the ABM control registers are included in the boundary-scan register, which can therefore be loaded and unloaded through the TAP as with the DBMS.

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