open-source-silicon.dev

questions about the empty project wrapper:
1. is there a reason why I can't use the digital pins for analog IO?
2. io_clamp_low = GND and io_clamp_high = 1.8V right?
3. Why do some analog pins not have a clamp?
4. Do I connect VSSD2 = GND and VCCD2 = 1.8V?
5. What are the "tiny" pins for? If I'm doing analog design I probably don't need them, right?

<@U03QSBWN0TU> I’m not an expert, but this is my current understanding. (Hopefully someone will correct be if I’m wrong).
1. The digital gpio do have an analog mode, but the analog pins have some resistors and esd structures connected. The 11 analog pin have straight connections - you are responsible for esd.
2. You don’t need to connect these in `user_analog_project_wrapper`. These are inputs from chip_io that you can use if needed for the above mentioned esd. Don’t know what `io_clamp_high` is connected to but yooou should be able to trace it in caravel.
3. None of the analog io pins have clamps. The `io_clamp_low` and `io_clamp_high` are inputs from I think 2 pads that you can use in `user_analog_project_wrapper`.
4. vssd2 and vccd2 are inputs. If you don’t use them in `user_analog_project_wrapper` , leave them open.
5. Looks like the tiny pins that you’re referring to are the control signals for the gpio’s. If you don’t use the gpios, make sure to configure them to be mgmt controlled in your `user_defines.v`. Then the pins can be left open.

so the VSS/VCC pins are just power inputs?

also for ESD protection how do I do it? diodes to ground/vcc?

(1) To elaborate on what Mitch said, the digital GPIO can be used for analog if your analog signal falls within the 3.3V range of the pad, and if your analog signal works with a frequency under about 50MHz.  The lower GPIO pads (0 to 6) are reserved for critical system functions like the SPI, debug, and UART, and so the analog pins are not available to the user wrapper.  But you can use any of the other GPIO for analog signals.

(2) You only need to connect the clamps if you are using those pads as an additional power supply input.  Power supplies need clamps to prevent damage from ESD events.  The I/O clamps are nominally for 3.3V operation (i.e., connect io_clamp_high to 3.3V), but since the I/O is specified to operate from 1.8V to 5.5V, then presumably the clamps work okay at 1.8V.

(3) The clamp circuits are not appropriate for anything that is _not_ a power supply, or at least a DC circuit.  Attaching an AC signal to a clamp will cause the clamp to activate, since the clamp is edge-triggered.  The clamp is also extra capacitance near the pad, so to leave most of the analog pads to be available and useful for high-speed analog signaling, the number of pads with clamps has been purposefully limited to just a few.
(4) All VCC* and VSS* are externally-supplied power supplies.  You don't need to connect them to any supply internally.  They can be connected in various ways on the demonstration board.

You can look in the <#C027VASS00L|ieee-sscs-dc-21q3> channel for a reference to a great talk that Boris Murmann gave on ESD circuits, a talk that was specifically directed toward people designing circuits using the analog pads on the Caravan chip.  Diodes to power and ground are a good "first defense" against ESD events.