1. TOPIC : Design of Scan Storage Cells UNIT 4 : Design for testability Module 4.3 Scan Architectures and Testing

2. Introduction A scan cell has both normal operation mode and test mode. It is chosen by using a MUX controlled by N/T signal. It can also be done by a two-clock system. The scan cell can be implemented by using many kinds of flip-flops. We will discuss how a scan storage cell (SSC) is built.

3. Basic Unit in SSC To observe the output, it should be stored for some time period. We use a basic D-latch for this purpose. It is the basic unit in many scan storage cells. D-Latch using NAND gates Representation of D-Latch

4. SSC Design A 2-port clocked master slave flip flop is joined with a MUX to form a SSC. Multiplexed Data flip-flop (MD-FF)Representation

5. Contd … MUX is used to select the mode of operation (normal or test) which is controlled by N/T. Master slave flip-flop is used to observe the output. When N/T = 0, normal data enters from port D1. N/T = 1, scan data (test data) enters at port D2. A single clock is used here, some times different clocks can be used for normal mode and scan mode.

6. SSC design with dual clocked flip-flop Instead of using N/T, two clock pulses can be used which are used to switch the circuit to normal and test modes. This is similar to a MUX with two control lines ck1, ck2. ck1  normal system clock, ck2  scan data input clock. 2-port dual clocked flip-flop (2p-FF)Representation

7. Multiplex Data Shift Register Latch(MD-SRL) It consists of latches L1, L2 with a multiplexed input data. Multiplex data shift register latch (MD-SRL)Representation

8. Contd … Since each latch has its own clock, this cell is not a flip- flop. A MUX is used with N/T as the control signal. D1  normal data D2  scan data

9. 2 Port Shift Register Latch (2P-SRL) To avoid the delay introduced by the MUX, a new design can be used as shown in figure.

10. Contd … It is the NAND gate equivalent to the shift register latch used in a level sensitive scan design (LSSD) methodology employed by IBM. ^D1  normal data input D2 or Si  scan data input CK1 (C)  normal system clock CK2 (A)  scan data input clock CK3 (B)  L2 latch clock CK1, CK2 can be NORed and can be used as clock to L2. 2P-SRL Symbol