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Circuit Description

A BILBO register (built-in logic block observer) combines normal flipflops with a few additional gates to provide four different functions. The example circuit shown in the applet realizes a four-bit register. However, the generalization to larger bit-widths should be obvious, with the XOR gates in the LFSR feedback path chosen to implement a good polynom for the given bit-width.

When the A and B control inputs are both 1, the circuit functions as a normal parallel D-type register.

When both A and B inputs are 0, the D-inputs are ignored (due to the AND gate connected to A), but the flipflops are connected as a shift-register via the NOR and XOR gates. The input to the first flipflop is then selected via the multiplexer controlled by the S input. If the S input is 1, the multiplexer transmits the value of the external SIN shift-in input to the first flipflop, so that the BILBO register works as a normal shift-register. This allows to initialize the register contents using a single signal wire, e.g. from an external test controller.

If all of the A, B, and S inputs are 0, the flipflops are configured as a shift-register, again, but the input bit to the first flipflop is computed by the XOR gates in the LFSR feedback path. This means that the register works as a standard LFSR pseudorandom pattern generator, useful to drive the logic connected to the Q outputs. Note that the start value of the LFSR sequence can be set by shifting it in via the SIN input.

Finally, if B and S are 0 but A is 1, the flipflops are configured as a shift-register, but the input value of each flipflop is the XOR of the D-input and the Q-output of the previous flipflop. This is exactly the configuration of a standard LFSR signature analysis register.

Because a BILBO register can be used as a pattern generator for the block it drives, as well provide signature-analysis for the block it is driven by, a whole circuit can be made self-testable with very low overhead and with only minimal performance degradation (two extra gates before the D inputs of the flipflops).

(The original reference for the BILBO register is B.Könemann, J.Mucha, G.Zwiehoff, Built-In Logic Block Observation Techniques, Proc. IEEE Test Conference, 1979, p.37-41.)