4 ms·
The resistor compensating the manufacturing process differences reminds me of when I worked on the 3DFx Voodoo and there was a chain of transistors that sat inl
by djmips 6y ago
The resistor compensating the manufacturing process differences reminds me of when I worked on the 3DFx Voodoo and there was a chain of transistors that sat inline with the clock but you could select which output would be sent to the remote TMUs which were clocked by this line. Code in the start up would draw textured test patterns and examine the Frame buffer to adjust the clock timing by nanoseconds using the chain of transistors. This was actually necessary because of variances in the manufacturing. When 3DFx switched to a completely new chip maker our boards failed and we had to fix our startup code because it didn't have enough margin. Thankfully there were more transistors in the chain we weren't using before. Crisis averted. The reason our boards were susceptible than the reference design is that we had one of our TMUs slightly further away from the FBI.
- kens 6y agoIt's interesting to hear that the 3DFx adjusted the clock that way. Coincidentally, I was just reading about similar clock adjustment in the Pentium II and 4. They had "adaptive deskewing", where a phase comparator would adjust the clock delay as needed. It sounds like 3DFx did the adjustment at startup, but the Pentium did it during use so it could compensate for temperature drift. The Itanium 2 had similar deskewing, except the value was set during manufacturing by blowing fuses. Source: "CMOS VLSI Design", page 806.
- ChuckNorris89 6y agoIIRC Intel does similar but way more advanced automatic deskewing black magic in the thunderbolt controllers. That's how they can carry high speed PCIe signals so effortlessly across your average copper cable(it was originally supposed to be optical).
- p_l 6y agoThat's somewhat standard part of transceivers with addition that PCI-E lower layers implement forced skew themselves - even on the motherboard. While I don't exactly know the case with Thunderbolt, "normal" Display Port uses PCI-E physical layer, just unidirectional and with different protocol on top. On networking equipment, the necessary signal corrections are part of why (other than DRM) it's more expensive to use full transceivers that accept cables, vs. fixed-length cables with fixed transceivers vs. direct-attach cables which have minimal logic for signal quality.
- foobiekr 6y agoThis is an amazing story.Thank you for sharing it.