
JTAG BOUNDARY SCANS
OVERVIEW
SEC ASIC
7-1
STD80/STDM80
OVERVIEW
A board test is typically achieved by using in-circuit test techniques. However, in-circuit test techniques
demonstrate significant limitations for Surface Mount Technology (SMT) and Fine Pitch Technology (FPT)
boards. The pin and pad spacings getting tighter make it difficult to test boards with traditional methods
economically and reliably.
A boundary scan design reduces the cost of a function test. A boundary scan design circuitry allows boards
to be tested using the equivalent in-circuit test technique without bed-of-nails fixture. In recognition of the
increasing acceptance of the boundary scan test, IEEE and JTAG (Joint Test Action Group) developed IEEE
Standard Test Access Port and Boundary Scan Architecture (IEEE Std 1149.1).
A boundary scan technique requires to place a boundary scan cell adjacent to each component pin so that
signals at component boundaries can be controlled and observed using scan testing principles. Each
boundary scan cell for a given component is able to capture data from an input pin or from its internal logic,
and to drive its internal logic or an output pin. Boundary scan cells for the pins of a given component are
interconnected so as to form a shift-register chain around the border of the design, known as a boundary
scan register. Boundary scan registers for individual components can be connected in series to form a
single path through the complete design as shown in the figure 9-1. Alternatively, a board design can
contain several independent boundary scan paths that allow individual components to be tested as well as
the interconnections between components.
To test component interconnections, test data are first shifted into all boundary scan register cells
associated with component output test pins. Test data are then loaded into parallel inputs of boundary scan
cells associated with input pins through the component interconnections, and data captured in these cells
are shifted out from the boundary cells for evaluation. For an individual component test, a boundary scan
register is used to isolate on-chip system logic from stimuli received from surrounding components. An
actual test can be performed through the boundary scan path or the built-in self-test hardware.
Figure 7-1. Board Design for Boundary Scan
LOGIC
LOGIC
LOGIC
LOGIC
DATA IN
SERIAL
DATA OUT
SERIAL TEST INTERCONNECT
SYSTEM INTERCONNECT
SERIAL