topic : test points unit 4 : design for testability module 4.1: basics of dft
Post on 20-Jan-2016
214 Views
Preview:
TRANSCRIPT
TOPIC : Test PointsTOPIC : Test Points
UNIT 4 : Design For Testability
Module 4.1: Basics of DFT
What are test points?What are test points?Points that are used to give test inputs to a
block in the circuit and observe their outputs.
Employ test points to enhance controllability and observability.
Two types of test points◦ Control points◦ Observation points
Control points are primary inputs used to enhance controllability.
Observation points are primary outputs used to enhance observability.
Contd …Contd …Different ways to introduce test points are:Use jumpersAdd new control inputs
◦ 0-injection◦ 1-injection◦ 0/1-injection
Use multiplexer
JumpersJumpersSignal ‘G’ can be routed to 2pins A & A’ and
connected by a removable wire. This is called a jumper.
For testing, the external wire is removed and the wire stays connected during normal operation.
Original Circuit Circuit using jumper
Contd …Contd …
After removing the wire, the external test equipment can
Test C2 by applying arbitrary signals to A’ and observing the primary output.
Test C1 by applying arbitrary signals at the primary inputs and observing the outputs at A.
Thus A acts as an observation point and A’ acts as a control point.
0 – injection circuit0 – injection circuitA new control input CP has been added to gate G
to make it G*.
If CP=0, original and modified gates will be the same and the circuit operates in normal mode.
If CP=1, OP is forced to ‘0’ and ‘0’ is injected to C2.
Since ‘0’ is injected we call it as 0-injection circuit.
1-injection circuit1-injection circuitSimilar to 0-injection, a 1 can also be injected.A control point is added to a OR gate to inject 1.
If CP=0, the functioning of the OR gate doesn’t change and the circuit works in normal mode.
If CP=1, it deactivates the normal operation and OP is forced to 1 which will be injected to C2.
Since a 1 is injected, we call it as 1-injection circuit.
0/1 – injection circuit0/1 – injection circuitAdditional input CP1 has been added to gate
G and another gate G’ has been added.
If CP1=CP2=0, then the circuit works in normal mode.
CP1=1 sets G* to 0, then G’=CP2, CP2 can be any value.
Thus G’ can be controlled to either 0 or 1 and this value can be injected to C2. Hence we call it as 0/1-injection circuit.
Test points using MUXTest points using MUXA multiplexer is added to the circuit.
CP2 is the select line. If CP2=0, then the circuit works in normal
mode. If CP2=1, G’=CP1 which can be any value.Hence a 0/1 can be injected into C2 to test.
General way to inject 0/1General way to inject 0/1For a given signal line G, Insert an AND/NOR gate for 0-injection Insert a NAND/OR gate for 1-injection Insert series of gates, example NOR-NOR for
0/1 injection.
The problem in the above cases is the number of output pins and input pins are increased.
top related