documentation

README.md

@@ -8,3 +8,32 @@ via parameters.
 This Program is subject to the terms of the Mozilla Public License,
 v.2.0. A copy of this license may be found in the file `COPYING`. You
 can obtain one at https://mozilla.org/MPL/2.0/.
+
+## Tests
+
+Go into `tests` and run `make`. To rerun tests, run `make clean` followed
+by `make`.
+
+The test for each mode generates a `.vcd` file which you may view using
+[GTKWave][1]. You can use this to gauge which SPI mode is appropriate for
+your device.
+
+[1]: https://gtkwave.sourceforge.net/
+
+## SPI Modes
+
+Modes are denoted by `modePH`, where `P` is the polarity (0 for normal,
+1 for inverted) and `H` for phase:
+
+* `H = 0` means the device reads on a rising edge and writes on a falling
+  edge.
+* `H = 1` means the device reads on a falling edge and writes on a rising
+  edge.
+
+Although these modules support all SPI modes, they are labeled slightly
+differently from other SPI modes. The phase factor is denoted in terms
+of falling and rising edges, not in terms of leading and trailing edges.
+This means that polarity also flips the phase term, so a mode 3 device
+is a mode 10 device. Devices with regular clock polarity are unaffected,
+so a mode 0 device is a mode 00 device, and a mode 1 device is a mode
+01 device.