Updated the documentation to include a description of the power-on-reset
circuit and the completed user project example.
diff --git a/docs/source/index.rst b/docs/source/index.rst
index 6b8efef..a5a9e4c 100644
--- a/docs/source/index.rst
+++ b/docs/source/index.rst
@@ -87,7 +87,21 @@
User Project: Power on Reset
----------------------------
-> :construction: Under construction :construction:
+This is an example user analog project which breaks out the power-on-reset
+circuit used by the management SoC for power-up behavior so that the circuit
+input and output can be independently controlled and measured.
+
+The power-on-reset circuit itself is a simple, non-temperature-compensated
+analog delay calibrated to 15ms under nominal conditions, with a Schmitt
+trigger inverter to provide hysteresis around the trigger point to provide
+a clean output reset signal.
+
+The circuit provides a single high-voltage (3.3V domain) sense-inverted reset
+signal "porb_h" and complementary low-voltage (1.8V domain) reset signals
+"por_l" and "porb_l".
+
+The only input to the circuit is the 3.3V domain power supply itself.
+
Verilog Integration
-------------------
@@ -100,6 +114,16 @@
user space utilities provided by caravel like IO ports, logic analyzer
probes, and wishbone bus connection to the management SoC.
+The verilog modules instantiated in the wrapper module should represent
+the analog project; they need not be more than empty blocks, but it is
+encouraged to write a simple behavioral description of the analog circuit
+in standard verilog, using real-valued wires when necessary. This allows
+the whole system to be run in a verilog testbench and verify the connectivity
+to the padframe and management SoC, even if the testbench C code does nothing
+more than set the mode of each GPIO pin. The example top-level verilog code
+emulates the behavior of the power-on-reset delay after applying a valid
+power supply to the circuit.
+
Building the PDK
================
@@ -165,7 +189,58 @@
Analog Design Flow
===================
-> :construction: Under construction :construction:
+The example project uses a very simple analog design flow with schematics
+made with xschem, simulation done using ngspice, layout done with magic,
+and LVS verification done with netgen. Sources for the power-on-reset
+circuit are in the "xschem/" directory, which also includes a schematic
+representing the wrapper with all of its ports, for use in a testbench
+circuit. There are several testbenches in the example, starting from
+tests of the component devices to a full test of the completed project
+inside the wrapper.
+
+There is no automation in this project; the schematic and layout were
+done by hand, including both the power-on-reset block and the power and
+signal routing to the pins on the wrapper.
+
+The power-on-reset circuit itself is simple and is not compensated for
+temperature or voltage variation. When the power supply reaches a
+sufficient level, the voltage divider sets the gate voltage on an nFET
+device to draw a current of nominally 240nA. The testbench
+"threshold_test_tb.spice" does a DC sweep to find the gate voltage that
+produces this value. Next, a cascaded current mirror divides down the
+current by a factor of (roughly) 400. The testbench current_test.spice
+checks the current division value. Finally, the output ~600pA from the
+end of the current mirror is accumulated on a capacitor until the value
+trips the input of the 3.3V Schmitt trigger buffer from the
+sky130_fd_sd_hvl library. The capacitor is sized to peg the nominal
+time to trigger at 15ms. The schematic "example_por_tb.sch" sets up
+the testbench for this timing test.
+
+The output of the Schmitt trigger buffer becomes the high-voltage
+output, and is input to a standard buffer and inverter used as
+level shifters from the 3.3V domain to the 1.8V domain, producing
+complementary low-voltage outputs.
+
+The user project is formed from two power-on-reset circuits, one of
+which is connected to the user area VDDA1 power supply, and the other
+of which is connected to one of the analog I/O pads, used as a power
+supply input and connected to its voltage ESD clamp circuit. The
+3.3V domain outputs are connected directly to GPIO pads through the
+ESD (150 ohm series) connection. The 1.8V domain outputs are connected
+to GPIO pads through the usual I/O connections, with the corresponding
+user output enable (sense inverted) held low to keep the output always
+active.
+
+The C code testbench is in "verilog/dv/mprj_por/mprj_por.c" and only
+sets the GPIO pins used to the correct state (user output function).
+The POR circuit outputs are monitored by the testbench verilog file
+"mprj_por_tb.v" which will fail if the connections are wrong or if
+the behavioral POR verilog does not work as intended.
+
+Note that to properly test this circuit, the GPIO pins have to be
+configured for output to be seen and measured, implying that the
+management SoC power supply must be stable and the C program running
+off of the SPI flash before the user area power supplies are raised.
Running Open-MPW Precheck Locally
