RTimothyEdwards
commented
2 years ago We need additional documentation, as some of the specs for the I/O cells in particular are in SkyWater documentation that did not get into the public repository.
This diagram explains the data connections to the pads on the caravan chip. For the eleven pads across the top, there is a straight-through wire from the pad to the core, with no ESD protection. Those are the io_analog pins. The GPIO pads are more flexible; they have digital input and output buffers, but those can both be turned off by configuration, and analog signals can be connected via gpio_noesd (straight wire through from the pad) or gpio_analog (wire to the pad in series with a 150 ohm resistor). Both of these analog connections have ESD protection diodes---the "noesd" in the name refers to the fact that there is no additional machine-model (MM) ESD protection. Do not use gpio_noesd unless your circuit cannot tolerate the 150 ohm series resistance (and if you do use it, be aware that your chip will be considerably more ESD-sensitive).
jeffdi
It would be great to know the structure of the
gpio_analog,gpio_noesdandio_analogpins on the pad-frame. Knowing the load resistance or capacitance present would help in designing efficient measurement circuits and buffers for analog designs.