Pi-Zero: OTG configuration (#180)

Document how to setup the Pi Zero with a special "OTG" cable to power the Pi from the AxiDraw's servo regulator and connect the Pi's Micro USB to the AxiDraw's Micro USB. Signed-off-by: Trammell Hudson <[email protected]>
nornagon · Nov 15, 2023 · 0b42938 · 0b42938
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diff --git a/README.md b/README.md
@@ -34,6 +34,36 @@ $ sudo apt-get install -y nodejs
 
 and then proceed as above :) If you connect to the raspberry pi over ssh, you might want to run the `saxi` server inside a tmux or screen session to have it stay running even if your ssh session disconnects.
 
+#### Raspberry Pi Zero OTG
+
+![Pi Zero on an AxiDraw with a Y-shaped USB cable](docs/pi-zero.jpg)
+
+For the Pi Zero you can make a USB "OTG" cable out of two Micro-B cables and two 0.1" headers
+to tap into the AxiDraw's 5V servo supply to power the Pi.  This makes for a more compact
+installation without the need for an additional wall-wart.
+
+
+```
+           +------ Center pin on servo rail
+           | +---- Ground pin on servo rail
+           | |
+           | |
+Red   -----+-|---- Red
+Black -------+---- Black
+White ------------ White
+Green ------------ Green (sometimes Blue)
+```
+
+The Pi will also need to have the `dr_mode=host` parameter set in
+`config.txt` to force host mode, since normal USB Micro cables do not
+include the `ID` pin that would be used to signal that it is an OTG
+connection.
+
+```
+echo dtoverly=dwc2,dr_mode=host | sudo tee -a /boot/config.txt
+```
+
+
 #### CORS
 If you want to connect to saxi from a web page that isn't served by saxi
 itself, you'll need to enable

diff --git a/docs/pi-zero.jpg b/docs/pi-zero.jpg