diff --git a/tech/SPI.wiki b/tech/SPI.wiki
new file mode 100644
index 0000000..ebd8e1d
--- /dev/null
+++ b/tech/SPI.wiki
@@ -0,0 +1,32 @@
+= SPI =
+
+SPI or Serial peripheral interface is the standard often used as a form of
+interchip communication on boards, and in embedded devices. 
+
+See https://www.youtube.com/watch?v=MCi7dCBhVpQ
+
+== Layout ==
+
+Each device, the master and slave (often microcontroler as master and
+peripheral/sensor as slave) have four pins
+
+* SCK - Serial Clock
+* CS  - Chip select (active low)
+* SDI - Serial data in
+* SDO - Serial data out
+
+NOTE serial data * is from the persepctive of the device (SDI on master is
+connected to SDO on slave and vice versa).
+
+SDO on the master to SDI on the peripherial is often called MOSI (Master out
+Slave in), and the inverse is often called MISO (Master in Slave out).
+
+Some chips may only have a MOSI, as some devices do not create data to send
+back to the master.
+
+== Sending data ==
+
+On the *rising edge* of the clock, each device reads what is currently set on
+the respective input port. The chip must also have its output set before the
+rising edge, to ensure that the device will receive the data it intends to
+send. All of this only occurs when CS is set low.
diff --git a/tech/embedded.wiki b/tech/embedded.wiki
index 5debd6c..3c9e726 100644
--- a/tech/embedded.wiki
+++ b/tech/embedded.wiki
@@ -16,7 +16,7 @@ user, sometimes so that they don't even know there is a computer
   - Pi Pico
 * [[ARM]] Processor type
 
-=== Line of products ===
+== Line of products ==
 
 * [[ATMega328P|Arduino Nano]]
 * [[ESP32]]
@@ -41,16 +41,22 @@ user, sometimes so that they don't even know there is a computer
 
 * [[nRF24L01]]
 
+Also see [[Ham]]
+
 ==== Power regulation ====
 
 * [[TP4056]]
 * [[TPS76933DBVR]]
 
-=== Programming ===
+== Programming ==
 
 * [[../lang/C|C]]
 * [[../lang/C++|C++]]
 * [[arduino_techniques|Techniques]]
 * [[cross_toolchain|Arduino Cross-Toolchain setup]]
 
+== Communication ==
+
+* [[SPI]]
+
 [[index]]