tornado/doc/README: describe some technical parameters

tornado/doc/README

@@ -0,0 +1,56 @@
+Antorcha vs. Tornado feature comparison
+---------------------------------------
+
+Number of LEDs		32			64
+Pixels			16			64
+LED pitch		8 mm			5 mm
+1 arc sec / pixel (1)	28 m			17 m
+1 arc sec / text (2)	70 m			90 m
+LED line length		25 cm			32 cm
+Total stick length	63 cm			53 cm (TBD)
+Outer radius		~ 90 cm			~ 80 cm (3)
+Visible width		~ 100 cm		~ 110 cm (3)
+Sweeps per second	~ 4, bidirectional	~ 3, unidirectional
+Sweep speed (avg.)	~ 8 m/s			~ 15 m/s (3)
+
+(1) http://en.wikipedia.org/wiki/Visual_acuity
+(2) 2 text lines, 1/12 display height
+(3) for chain length = 30 cm
+
+
+Weight
+------
+
+Al stick: 52.5 g / 52.7 cm = ~ 1 g/cm
+Chain:	44 g / 19 links = ~ 2.3 g/link
+	305 mm / 21 links = 14.5 mm/link
+	1.6 g/cm
+Attachment (screw, washers, nut): 2.3 g
+End cap: 1.54 g
+Cable:	1.74 g / 382 mm
+	group of 3: ~ 0.14 g/cm
+LED PCB: 4.1 g
+CPU PCB: 4.5 g
+
+Not included in the weight calculation:
+- glue
+- adhesive tapes
+- isolation tapes
+- cable binders (if any)
+- heat-shrink tubing
+- cables/wires for inter-board connections
+
+
+Update rate
+-----------
+
+For square pixels we need one update every 5 mm. With the edge moving
+at 15 m/s, that's one update every 1/3000 s. If we assume that,
+including overhead, each 64 bit shift and latch takes about 100 clock
+cycles, the serial bus needs to run at no less than 300 kHz.
+
+sclk-1123.png shows the ~250 ns clock pulse generated by the current
+test firmware. CH2 (blue) is signal entering the first LED module,
+CH1 (yellow) is the signal exiting the last module. The absence of
+degradation in a 250 ns pulse suggests that we can go as fast as the
+MCU is able to.