[libre-riscv-dev] NLNet018TV documentation

[Staf Verhaegen]

Goodday all,

I did tape-out the TSMC 0.18µm test chip around a week ago and am now
hard at work in documenting was is on the chip. I just pushed first
version of the documentation and can be found in the designs/NLNet018TV
 directory on my SnowWhite repo on gitlab. Currently it only discusses
what is on the design; not why these structures are there with this
design. This is planned for later. Next step is to add a test plan and
then blog about it.

Most significant change is that I did not put a full SRAM block on the
design but single SRAM cells only. This was both due the limited
availability of IO pins and the time pressure of the tape-out deadline.
Instead more focus was put on the design of the IO cells which is
actually the main reason of this test chip.

I did currently have three different drive strengths for the source and
sink drivers next to the pull-up/pull-down functionality.
The highest drive strength of 230mA is meant for driving resistive
loads like for example LEDS. The real current will most of the time be
limited by an external resistor in order to limit the heating of the
chip. This is one the things that will be measured during testing.
These test chips are wirebonded semi-manually and to ease it a pitch of
90µm between the IO cells is used. The 230mA driver strength is the
result of filling up the total width of the IO cell with drivers.
Depending on the number of outputs for the prototype we may negotiate a
smaller pitch but this will also reduce the drive strength of this big
driver or alternatively the IO cell may need to be made higher.
The two lower drive strength are meant to drive capacitive loads. They
can be either used alone or combined giving possibility of having 10mA,
20mA or 30mA (e.g. 10mA + 20mA) drivers. It allows to configure them as
having the driver sink or source current or both. Reason I choose these
drive strengths is that the 10mA corresponds with one transistor needed
to fulfill the ESD design rules and in simulation I did see acceptable
output over/undershoot even with minimal capacitive load. From
simulation a 40mA driver gave significant overshoot/undershoot and
oscillation even with high capacitive loads.
The design of the IO cell is quite flexible and can still be adapted if
there would be special needs for the prototype. So I am interested to
know what changes you think are needed for the prototype.

greets,
Staf.