FPGA sensitivities

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I have a time-critical thing where the signal passes through an XC7A15
FPGA and does a fair lot of stuff inside. I measured delay vs some

1.8 aux   no measurable DC effect
3.3 vccio no measurable DC effect

2.5 vccio ditto (key io's are LVDS in this bank)

+1 core   -10 ps per millivolt!

If I vary the trigger frequency, I can see the delay heterodyning
against the 1.8V switcher frequency, a few ps p-p maybe. Gotta track
that down.

A spritz of freeze spray on the chip had practically no effect on
delay through the chip, on a scope at 100 ps/div.

I expected sensitivity to core voltage, so we'll make sure we have a
serious, analog-quality voltage regulator next rev.

The temperature thing surprised me. I was used to CMOS having a
serious positive delay TC. Maybe modern FPGAs have some sort of
temperature compensation designed in?

We also have a ZYNQ on this board that crashes the ARM core
erratically, especially when the chip is hot. It might crash in maybe
a half hour MTBF if the chip reports 55C internally; the FPGA part
keeps going. At powerup boot from an SD card, it will always configure
the PL FPGA side, but will then fail to run our application if the
chip is hot. We're playing with DRAM and CPU clock rates to see if
that has much effect.

Re: FPGA sensitivities
On 2020-09-25 15:16, John Larkin wrote:
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Yecch, good to know--keeping the drift down to 1 ps requires the 1V  
supply to be stable to 100 ppm total.  I don't think I've ever needed to  
use four-wire sensing on a logic supply, but you're probably going to  
have to.


Phil Hobbs

Dr Philip C D Hobbs
Principal Consultant
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Re: FPGA sensitivities
On Fri, 25 Sep 2020 16:49:51 -0400, Phil Hobbs

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I don't expect to keep the delay stable to 1 ps over temperature.
Below 1 ps/oC would wipe the competition. I do want to get the jitter
down into the single digits of ps RMS.

I boogered the +1 volt (Zynq core) supply voltage up to 1.1 volts and
the ARM crash thing went away. Or the crash temperature went way up.
So we have a timing problem.

My engineers are working from home but one is burning me a new SD card
to try, with slower clocks in the ARM and DRAM. I'll pop over soon and
pick it up. She lives in a tiny rent-controlled apartment above
Dolores Park. Her next-door neighbor on that block is one of the
richest people in the world.

Re: FPGA sensitivities
On Friday, September 25, 2020 at 3:16:19 PM UTC-4, John Larkin wrote:
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You don't say how much the total delay is or what your circuitry is other t
han "a fair lot of stuff".  I'm pretty sure the FPGA makers focus on minimi
zing worst case delay rather than making it more even with temperature.  No
 one would have much use for that when push comes to shove.  If it works, j
ust barely, at high temperature, what is the point of maintaining a bare ti
ming margin at lower temps rather than letting it speed up?  

It may well be that when going on and off chip the I/O delays make up the l
ion's share of the timing and they may not behave the same as the bulk sinc
e they are mostly about large structures.  

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Hardly.  More like they focus on optimizing clock speeds so raw on chip log
ic and routing speed.  Try using a clock in your design and see how fast it
 will operate.  We did that once when the design software was marginal and  
did not properly estimate delay on a heavily loaded net.  It would report m
eeting the clock speed requirement when it was actually failing.  Cold spra
y would make it work.  In the end we had to test using a heat plate to be a
ble to ship a product.  

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What a PITA.  Timing/heat problems are awkward to debug when the tools are  
not of much help.  

Are you not cooling it enough to keep it from crashing?  


  Rick C.

  - Get 1,000 miles of free Supercharging
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Re: FPGA sensitivities
On Fri, 25 Sep 2020 12:16:07 -0700, John Larkin

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Fixed both problems.

Jitter: replaced the 1.8V Vccaux switcher with a linear regulator.

Temperature-dependant crashing: I found an oscillation on the Zynq 1v
core power supply, about 100 mV p-p and 80 KHz. Putting a lot more
capacitance at the switcher output kills that and makes the crash go
away. The regulator design followed a chart in the LTM8078 data sheet.
A Spice sim with the original values looks stable, no oscillation and
a clean load-step recovery.

There are other indications that ADI's Spice model of the LTM8078 is
less than perfect. I think ADI is struggling to add a lot of new parts
to the LT Spice libraries. Mike E in an interview suggested that
rushing them out was compromising quality. Then he quit.

Glad I fixed this this way. Guys were snooping the AXIbus and Linux at
great expense and no progress.

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