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In caravel analog example, why those io_clamps are "conected" to metal3-rmetal3-metal3 and connecte...
# sky130
h
In caravel analog example, why those io_clamps are "conected" to metal3-rmetal3-metal3 and connected to the vssa1? And what the importance of rmetal?
w
Rmetal is the easiest way to do a net-tie. As spice / magic can have issues when two ports are connected directly
Rmetal for any given layer is the same material as metal but it is extracted as a resistor, so the two sides are on different nets.
Here it is used to connect the IO clamp to vssa while having it still be a separate net.
h
If i dont use any analog pins with clamps, do i need to connect clamps on vssa?
w
You are using the caravan harness, right? A few of the analog pins are more designed as power pins and thus they have the clamps.
🌍 1
I forgot the exact structure, this may be a question for @User . But you should be able to leave them floating.
You also probably want some ESD protection structure though. This analog pins don't have any protection beyond the clamp. And even then I don't think the clamp is suitable for all cases
h
In my case, i have 4 pins: in - digital vdd - power vss - ground out - analog of GHz values I think i dont neet those analog pins with clamps, or am i wrong?
t
vdd
should be connected to a clamp unless you are getting it from one of the standard supplies on Caravel/Caravan (vdda1 or vdda2 if it's 3.3V; or vccd1 or vccd2 if it's 1.8V).
To get GHz output, you may want to forego ESD protection on the output line, but then your chip will be extremely ESD sensitive. Anything you can add---reverse-biased diode, series resistance---that you can get away without compromising your bandwidth will be helpful.
h
If i use rmetal, it will change just the extraction? In real life, rmetalX=metalX?
t
Yes, rmetal is the same as metal except that it will break the metal line into two different nets in the extracted netlist, connected by a resistor. When doing LVS you will need to represent the metal resistor as a device in the schematic netlist.
h
Can i let io_clamps floating (if i dont use them)? If dont, should i conect them on vssa like example?
t
Yes, you can leave them floating.
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