# timing closure degraded

**URL:** <https://yosyshq.discourse.group/t/timing-closure-degraded/139>\
**Category:** Users\
**Created:** [April 16, 2026, 9:46am UTC](https://yosyshq.discourse.group/t/timing-closure-degraded/139 "2026-04-16T09:46:48Z")\
**Posts on this page:** 9\
**Page:** 1

<div class="post-metadata">

**Author:** ![tambewilliam](https://avatars.discourse-cdn.com/v4/letter/t/e19b73/32.png) [@tambewilliam](https://yosyshq.discourse.group/u/tambewilliam)\
**Post date:** [April 16, 2026, 9:46am UTC](https://yosyshq.discourse.group/t/timing-closure-degraded/139/1 "2026-04-16T09:46:48Z")

</div>

Between oss-cad-suite 20251204 (with nextpnr-0.9-43) and 20260415 (nextpnr-0.10-33),

something changed such that nextpnr-ecp5, for the same exact design that used to pass 48 MHz timing, now fails achieving only 30 MHz.

I bisected the issue to oss-cad-suite 20260331.

nextpnr version did not change from the day before, so the issue might be somewhere else.

Any idea why timing closure degraded so much ?

---

<div class="post-metadata">

**Author:** ![widlarizer](https://avatars.discourse-cdn.com/v4/letter/w/bbce88/32.png) [@widlarizer](https://yosyshq.discourse.group/u/widlarizer)\
**Post date:** [April 16, 2026, 12:38pm UTC](https://yosyshq.discourse.group/t/timing-closure-degraded/139/2 "2026-04-16T12:38:09Z")

</div>

Could be related to [ABC regression · Issue #5802 · YosysHQ/yosys · GitHub](https://github.com/YosysHQ/yosys/issues/5802)

---

<div class="post-metadata">

**Author:** ![tambewilliam](https://avatars.discourse-cdn.com/v4/letter/t/e19b73/32.png) [@tambewilliam](https://yosyshq.discourse.group/u/tambewilliam)\
**Post date:** [April 17, 2026, 5:47pm UTC](https://yosyshq.discourse.group/t/timing-closure-degraded/139/3 "2026-04-17T17:47:31Z")

</div>

Is there a runtime workaround that can be used ?

---

<div class="post-metadata">

**Author:** ![widlarizer](https://avatars.discourse-cdn.com/v4/letter/w/bbce88/32.png) [@widlarizer](https://yosyshq.discourse.group/u/widlarizer)\
**Post date:** [April 17, 2026, 8:16pm UTC](https://yosyshq.discourse.group/t/timing-closure-degraded/139/4 "2026-04-17T20:16:22Z")

</div>

Have you tried an earlier version, something like a month old?

---

<div class="post-metadata">

**Author:** ![tambewilliam](https://avatars.discourse-cdn.com/v4/letter/t/e19b73/32.png) [@tambewilliam](https://yosyshq.discourse.group/u/tambewilliam)\
**Post date:** [April 17, 2026, 8:36pm UTC](https://yosyshq.discourse.group/t/timing-closure-degraded/139/5 "2026-04-17T20:36:06Z")

</div>

I know exactly when the problem started occuring; but I like that there is improved LUT usage with the latest oss-cad-suite, just that timing closure is degraded.

I was hoping there is a way to work around the issue at runtime and benefit from the lower LUT usage.

---

<div class="post-metadata">

**Author:** ![KrystalDelusion](https://avatars.discourse-cdn.com/v4/letter/k/8baadc/32.png) [@KrystalDelusion](https://yosyshq.discourse.group/u/KrystalDelusion)\
**Post date:** [April 19, 2026, 9:50pm UTC](https://yosyshq.discourse.group/t/timing-closure-degraded/139/6 "2026-04-19T21:50:17Z")

</div>

> [@tambewilliam](#):
>
> I like that there is improved LUT usage with the latest oss-cad-suite, just that timing closure is degraded.

I suspect these two things may be related and that it’s probably not possible to get one without the other.

---

<div class="post-metadata">

**Author:** ![tambewilliam](https://avatars.discourse-cdn.com/v4/letter/t/e19b73/32.png) [@tambewilliam](https://yosyshq.discourse.group/u/tambewilliam)\
**Post date:** [May 8, 2026, 8:17pm UTC](https://yosyshq.discourse.group/t/timing-closure-degraded/139/7 "2026-05-08T20:17:18Z")

</div>

Is there by any chance an ETA when a fix would be available ?

Or I shouldn’t hold my breath waiting on a fix.

---

<div class="post-metadata">

**Author:** ![lofty](https://avatars.discourse-cdn.com/v4/letter/l/f0a364/32.png) [@lofty](https://yosyshq.discourse.group/u/lofty)\
**Post date:** [May 12, 2026, 7:40pm UTC](https://yosyshq.discourse.group/t/timing-closure-degraded/139/8 "2026-05-12T19:40:20Z")

</div>

The testcase given in issue 5801 behaves correctly with Yosys 0.65 after an ABC regression was fixed; @tambewilliam can you check your design with that?

---

<div class="post-metadata">

**Author:** ![tambewilliam](https://avatars.discourse-cdn.com/v4/letter/t/e19b73/32.png) [@tambewilliam](https://yosyshq.discourse.group/u/tambewilliam)\
**Post date:** [May 13, 2026, 3:45pm UTC](https://yosyshq.discourse.group/t/timing-closure-degraded/139/9 "2026-05-13T15:45:46Z")

</div>

Thank you,

I no longer see the timing closure degradation using oss-cad-suite 20260513, which comes with yosys 0.65+7