Improvements for the Tiny Tapeout analog switch:
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# analog-designu
Uri Shaked
03/30/2024, 6:56 PMImprovements for the Tiny Tapeout analog switch:
https://xschem-viewer.com/?file=https://github.com/TinyTapeout/tt_asw_1v8/blob/main/xschem/tt_asw_1v8.sch
Uri Shaked
03/30/2024, 6:56 PM@Tim Edwards moving the discussion here, so Sylvain will also be able to participate
Uri Shaked
03/30/2024, 6:56 PMTim's last suggestion was:
You need the switch gated by 3.3V, but you can put the switch circuit next to the pad and wire it up to 3.3V manually without requiring openlane to deal with it.
Uri Shaked
03/30/2024, 6:58 PMThe problem we're trying to solve: any accidental connection between the analog pins and 3v3 could kill the chip
t
tnt
03/30/2024, 6:59 PMWell feel free to do it, but I don't have time for that, it's not a trivial change and we don't even have a 3v3 rail anywhere close to those IOs yet. And there is no space for that so this all need to fit under the PDN rings basically without interfering with the routing path.
tnt
03/30/2024, 7:01 PMIf the issue is just "user error" and it will work fine if the user keep things under 1.8V then I just see that as a "nice to have".
u
Uri Shaked
03/30/2024, 7:01 PMI was suggesting, as a workaround, to put solder bridges that connect the analog pads to GND
Uri Shaked
03/30/2024, 7:02 PM(in the breakout board)
Uri Shaked
03/30/2024, 7:02 PMso user error isn't easy to make - you first have to remove those bridges before you can make a user error
t
tnt
03/30/2024, 7:02 PMI would definitely put weak pull downs on the break out board at least.
u
Uri Shaked
03/30/2024, 7:04 PMI think what Tim suggested was to get the 3v3 directly from the pad next to the switches, so it won't need another core ring
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tnt
03/30/2024, 7:04 PMwhat pad ?
u
Uri Shaked
03/30/2024, 7:04 PMperhaps something like this:
Uri Shaked
03/30/2024, 7:04 PMmessage has been deleted
Uri Shaked
03/30/2024, 7:05 PMred boxes running on Metal 5,
t
tnt
03/30/2024, 7:05 PMI'm not sure what this is supposed to show.
tnt
03/30/2024, 7:05 PMAnd where are those box connected to ?
u
Uri Shaked
03/30/2024, 7:05 PMand they get their power from vddio
t
tnt
03/30/2024, 7:06 PMhow ?
u
Uri Shaked
03/30/2024, 7:06 PM(which is supposedly available somewhere on the pad)
t
tnt
03/30/2024, 7:06 PMIt's not exposed to us.
tnt
03/30/2024, 7:06 PMSo yeah, it's "somewhere" but we have no clue of where and how to get there.
tnt
03/30/2024, 7:07 PMPretty sure this would also make all tests fails as we would ahve metal out of the allowed area.
tnt
03/30/2024, 7:09 PMFeel free to play with it if you have spare time, but as I said, I don't have time for that, especially when the proper solution is just to add the 3v3 support, adding 3v3 PDN and using 3v3 switch and that will solve the issue by itself.
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