Protocols&Interfaces

Electronics is about voltage and stuffs but as humans we use it for connect devices and devices connected need ways to communicate.

PWM

The acronym means Pulse Width Modulation and it's the simplest way to digitally encode a signal using a single wire; it's defined by the duty cycle, i.e. the percentage of the cycle that the signal is on (0% is always off, 100% is always on) and its switching frequency, that identify how many seconds each raising edge appears.

Its fourier transformed form is given by this coefficients ( $p$ is the duty cycle $0\leq p\leq 1$ )

$A_0 = K\cdot p$ $A_n = K\cdot {1\over n\pi}\left(\sin(n\pi p) - \sin\left(2n\pi (1 - p/2)\right)\right)$

PWM Distortion Analysis

UART and Serial

The simplest of protocols without needing a clock line, usually it's exposed as a 4 pins header but it's also possible to be exposed via already used IO port (like headphone socket or USB port via USB port multimedia switch).

http://wiki.openwrt.org/doc/hardware/port.serial
http://www.devttys0.com/2012/11/reverse-engineering-serial-ports/
Chapter about design of a USART interface

I2C

It uses only two wires, and allows to connect up to 1008 slave devices.

I2C in a Nutshell

SPI

It's a protocol with a clock line and a differentiation between devices that can be master (that provides clock) or slave. Although may there be only one master there is a signal (SS) that indicates which one of the (possible) multiple slaves must respond.

It has the disadvantage that the communication must be well defined in advance since the master must know how many clock cycle need to listen from the slaves.

SD Card

For example an SD Card use this protocol (see a link in the Bus Pirate page) how you can read here and here.

The pin CD stand for card detection: when is low the card is inserted into its socket.

Links

https://learn.sparkfun.com/tutorials/serial-peripheral-interface-spi
Reference for SD card protocol
SD Card Protocol
Interfacing Microcontrollers with SD Card
Interfacing Micro SD Card Module with Arduino
SPI and SD cards

PS/2

Protocol description

JTAG

It's a protocol used to debug hardware and uses the following pins that constitute the TAP, the Test Access Port:

TMS: Test Mode Select. This pin is used to cycle through the TAP-state machine.
TCK: Test Clock.
TDI: Test Data In. Serial input data to be shifted in to the Instruction Register or Data Register.
TDO: Test Data Out. Serial output data from Instruction Register or Data Register.

There are also some optional pins:

TRST: TAP reset, optional because is always possible to out the state machine to a start state
SRST: system reset, allows to reset the system

Usually on reference is indicated also the VTRef pin that indicates what is the voltage reference for the signals.

JTAG explained
How JTAG works
Debugging with JTAG
https://hackingbtbusinesshub.wordpress.com/2012/01/26/discovering-jtag-pinouts/
http://sun.hasenbraten.de/~frank/docs/mpc824x_JTAG.html
Header per SOC da farnell
Slide of a talk about finding JTAG's pinout
JTAGEnum Given an Arduino compatible microcontroller JTAGenum scans pins for basic JTAG functionality.
http://www.sodnpoo.com/posts.xml/pace4000_jtag.xml
https://www.youtube.com/watch?v=TlWlLeC5BUs
http://electronics.stackexchange.com/questions/53311/why-jtag-connectors-are-available-in-10pins-14pins-20pins-when-jtag-is-of-5pins
https://github.com/syncsrc/jtagsploitation
Video Blackbox JTAG Reverse Engineering [26C3]
Preparing Raspberry PI for JTAG Debugging
Black Magic Probe V2 is a JTAG and SWD Adapter used for programming and debugging ARM Cortex MCUs.
Using a JTAG in Linux Driver Debugging
Can I use JTAG to debug my program on top of embedded Linux?
The JTAG Interface: AN ATTACKER’S PERSPECTIVE
Re-enabling JTAG and Debugging the WRT120N
Locating JTAG pins automatically
The jrev tool tries to automate the process of reverse-engineering the board connectivity. It probes all pairs of JTAG-scannable pins on the board.
Practical JTAG: From 0 to 1
SWD – ARM’S ALTERNATIVE TO JTAG