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--- the_i2c_bus [2020/01/15 22:55] – [I²C in FRC] tell
+++ the_i2c_bus [2022/02/03 09:06] (current) – tell
@@ Line 8: / Line 8: @@
 Those two wires are called "SCL" and "SDA, for "serial clock" and "serial data."
-Some basic facts aboutI²C:
+Some basic facts about I²C:
-* It was originally intended to stay on the same PC board, but works over a few feet of cable
+  * It was originally intended to stay on the same PC board, but works over a few feet of cable
-* It runs at 100Kbps or 400Kbps
+  * It runs at 100Kbps or 400Kbps
-* Each "slave" device has an address, and each slave on the bus must have a unique address
+  * Devices only pull SDA and SCL low
-* Some devices have pins that can chose from several addresses.  Others are limited to one per bus because the address can't be changed.
+  * Pullup resistors are required on SDA and SCL, but most boards have the installed already.
+  * Each "slave" device has an address, and each slave on the bus must have a unique address.
+  * Addresses are 7 bits long, therefore there are only 128 possible addresses.
+  * Some devices have pins that can chose from several addresses.  Others are limited to one per bus because their address can't be changed.
+====== I²C Protocol ======
+The I²C standard defines the basic signaling on the two wires for sending data back and forth to the device.  The primitives available are:
+  * Claim the bus and start a transaction: pull SDA low while leaving SCL high
+  * Stop and release the bus: release SDA to go high, while leaving SCL high
+  * Send a byte (and get an acknowlegement)
+  * Read a byte
+This figure shows the basic idea: start, write one byte, acknowledge, stop:
+{{ :i2c-protocol.png |}}
+All transfers start as shown here, with the master sending one byte onto the bus containing the 7 bit address and one bit that indicates whether the next byte will be a write (master to slave) or a read (slave sends to master).
+A good, relatively simple explanation of data transfer is https://i2c.info/i2c-bus-specification
+These get combined in various ways depending on the device.  Reading the datasheet is a must while writing software for an I2C device.
 ====== I²C in FRC ======
 {{ :roborio-i2c.png|}}
-Since the introduction of the RoboRio, there has been an easy way to use I²C for FIRST robots.  The connector on the Rio looks like this.  The connector has four pins.  In addition to SCL and SDA, there is a pin for 3.3v power, and a pin for the common ground for signal and power.  This means that no external power source is needed or recommended for attaching an I2C device.
+Since the introduction of the RoboRio, there has been an easy way to use I²C for FIRST robots.  The connector on the Rio looks like this.  The connector has four pins, but otherwise looks like the common "PWM" connectors.
+It uses the same housings and headers that hold pins on 0.1 inch centers.
+In addition to SCL and SDA, there is a pin for 3.3v power, and a pin for the common ground for signal and power.  This means that no external power source is needed or recommended for attaching an I2C device.
 Around 2018-2019 we started to see packaged color and distance sensors from Rev Robotics and others.
@@ Line 25: / Line 49: @@
   * http://www.revrobotics.com/rev-11-1729/
+====== Software for I²C devices: start with Arduino ======
+Nearly every I²C device has unique requirements from the software to communicate with it. Fortunately, the success of the Arduino family of microcontroller boards means that almost every interesting I²C device has example code ready to use with the arduino board and software.
+This is often the best place to find such examples.  Reading the sample code can help you understand the part's datasheet.
+By 2021 there appeared FRC Java libraries for some of the I²C sensors that we've used.  But going back to the Arduino C code is necessary for some of the interesting ones that are less common in FRC.
 ====== I²C in other maker/hobby wiring ======
 ===== QWIIC =====
@@ Line 33: / Line 65: @@
 [[http://sparkfun.com | Sparkfun]] has many sensor boards and controllers that use a tiny 4-pin connector they call [[http://sparkfun.com/qwiic|QWIIC]].  It carries the SDA and SCL signals, ground, and +3.3v power.
-(picture)
+{{ :quicc-board-cable.png |}}
 Adafruit wants to call the same thing [[https://learn.adafruit.com/introducing-adafruit-stemma-qt|STEMMA-QT.]]
+===== Eastbots I2C Tester =====
+In 2019 we had some flaky i2c operation, so we built this wiring tester
+[[testing_i2c_wiring]]