Part of the megaproject area contains a 10-bit Potentiometric Digital to Analog Converter with an external 3.3 Volt rail voltage, 1.8 Volt digital voltage control, controlled digitally through the RISC-V logic analyzer. This work is built off of Ashutosh Sharma‘s work and ports the device from the OSU180 to Skywater130 with several minor improvements, Sharma’s repository is listed below. The fundamental idea is to divide the reference voltage into N different voltages between the range of V_ref+ and V_ref- for the N-bit Digital to Analog converter. The design contained here uses a string of polysilicon resistors in series to create a string DAC. The resistors are connected to digital switches in order to achieve an exact voltage at the output. The device was built through hierarchical sub-circuits and sub-layouts starting off at 2-bit, then 3-bit and so forth, incrementally reaching 10-bit. With a full scale of 3.3 V and a 10 bit resolution, additional work will be done in the future to increase resolution and power, and reduce latency.
This is the Google/EFabless/Skywater Caravel submission of an Analog Spiking Neuron Circuit. The submission also includes a SONOS transistor array.
The circuit in the original paper is in 130nm technology and has a vdd of 300mV. Skywater pdk is hybrid 180nm/130nm node where the minimum transistor length is 150nm. As a result vdd needs to be higher in order to get the circuit to work properly. In simulation 700mV seems to work well.
Spiking pattern at v_buff for i_in = 10pA DC
A 2x2 array of nfet sonos cells with transistor sizing 420nmx150nm.
To setup and install the repo for development:
export PDK_ROOT=(where pdks will be installed) cd $PDK_ROOT git clone https://github.com/efabless/openlane.git -b mpw-one-a cd openlane export OPENLANE_ROOT=$(pwd) make
2. Clone and uncompress the repo
git clone https://github.com/Bryce-Readyhough/caravel_UNCC_MPW_1.git make uncompress -j$nproc