Hold-Time Failure After Clock-Buffer Change
A 100 MHz synchronous datapath in an FPGA connects two flip-flops with a 4.2 ns maximum and 0.55 ns minimum combinational delay. The launching flop has a maximum clock-to-Q of 0.35 ns and a minimum of 0.20 ns; the receiving flop has a 0.15 ns setup time and a 0.25 ns hold time. After a clock-buffer change, the clock arriving at the receiving flop leads the launching flop by 0.60 ns. The board passes at 25 °C and 1.2 V, but fails intermittently at -40 °C and 1.32 V; the failure is a single bit that changes only when a particular data pattern is applied. Static timing at the slow corner reports 4.7 ns of setup slack. The design team proposes reducing the clock from 100 MHz to 50 MHz. The product must remain at 100 MHz, and the FPGA fabric has spare LUTs but no spare global clock buffers.
1. Compute the hold slack and the setup slack. Identify whether the failure is a setup or hold violation, and explain the temperature/voltage pattern.
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Hold slack = (minimum clock-to-Q + minimum data delay) − hold time − clock skew = (0.20 ns + 0.55 ns) − 0.25 ns − 0.60 ns = −0.10 ns. The path violates hold by 0.10 ns. Setup slack at the slow corner = clock period − maximum clock-to-Q − maximum data delay − setup time − skew = 10 ns − 0.35 ns − 4.2 ns − 0.15 ns − 0.60 ns = 4.70 ns, which matches the reported value. The failure is therefore a hold violation, not a setup violation. Hold violations worsen at the fast corner because delays shrink and skew effects can grow; -40 °C and 1.32 V are faster than 25 °C and 1.2 V. A data-dependent single-bit failure is also consistent with a marginal hold path: only the shortest path and the particular capture relationship fail.
2. Explain why reducing the clock frequency will not fix this failure, and propose a concrete fix that preserves 100 MHz operation.
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Hold timing is a same-edge check: the launched data must not arrive too early at the receiving flop after the same active clock edge. The clock period does not appear in the hold inequality, so lowering the frequency from 100 MHz to 50 MHz changes setup margin but leaves the -0.10 ns hold slack unchanged. The correct fix is to increase the minimum data-path delay, reduce the clock skew, or both. A small delay buffer or a routed detour in the data path can add 0.15–0.25 ns to the minimum delay. Because the same buffer also adds to the maximum delay, recheck setup: adding 0.20 ns changes hold slack to +0.10 ns and setup slack to 4.50 ns. Alternatively, adjust the clock tree so the receiving flop no longer leads by 0.60 ns. Do not rely on temperature or voltage screening; the margin is too small and the failure is intermittent.
3. The chosen fix adds 0.40 ns to both the minimum and maximum data-path delay. Recalculate both slacks and state whether the path is now robust enough for production.
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New hold slack = -0.10 ns + 0.40 ns = +0.30 ns. New setup slack = 4.70 ns − 0.40 ns = +4.30 ns. Both are positive, so the immediate violation is removed. However, +0.30 ns hold margin may still be thin once clock-tree uncertainty, on-chip variation, and board-to-board skew are included. A production sign-off should use the worst-case minimum delay, maximum skew, and the fast corner, with explicit derating rather than typical values. The 4.30 ns setup margin is comfortable at 100 MHz. Also confirm that the added delay is in the minimum path and is not removed by synthesis optimization; a dedicated delay cell or a keep attribute may be needed. Reducing the clock to 50 MHz remains unacceptable because the product requirement is 100 MHz and it would not address the hold check anyway.