Scan testing digital logic with differing frequencies of system clock and test clock

Claim 1

A test circuit comprising: Y scan flip-flops each having an input and an output, said Y scan flip-flops being connected serially in a sequence from a first scan flip-flop to an Y th scan flip-flop whereby the output of each of the first through (Y-1) th flip-flops is connected to the input of a scan flip-flop that is immediately next in sequence, said Y scan flip-flops being clocked by a system clock signal; feedback means connected between the output of the Y th scan flip-flop and the input of the first scan flip-flop for recirculating data in the scan flip-flops at the frequency of the system clock signal and for providing input data to the first scan flip-flop at the frequency of a test clock signal, wherein the ratio of the period of the test clock to the period of the system clock is equal to an integer M, said feedback means comprising: synchronizing means responsive to the system clock signal and the test clock signal for generating a synchronizing signal that is in a first logical state during every M th active transition of the system clock signal and in a second logical state between occurrences of said first logical state; serial-to-parallel shift register means having L registers each having an input and an output, said L registers being connected serially in a sequence from a first register to an L th register whereby the output of each of the first through (L-1) th register is connected to the input of a register that is immediately next in sequence, said L registers being clocked by the system clock signal; first selection means having inputs connected to the outputs of said serial-to-parallel shift register and the output of said Y th scan flip-flop for providing an output that is a replica of a selected one of said inputs; control means connected to said first selection means for providing control signals to select said selected one of said inputs, wherein a length of said shift register means is programmably set by said control means to thereby provide a variable extension of said sequence of scan flip-flops; second selection means having a first input connected to the output of said first selection means, a second input for receiving scan input data, and an output connected to the input of said first scan flip-flop, said selection means responsive to said synchronizing signal for providing a selection means output that comprises (a) a replica of the scan input when said synchronizing signal is in said first logical state, and (b) a replica of the output of said first selection means when said synchronizing signal is in said second logical state; and an output flip-flop clocked by the system clock and having an input connected to the output of said first selection means, said output flip-flop being periodically enabled pursuant to said test clock signal when said synchronization signal is in its first logical state.