In the digital world, where exist only 1 and 0 you have to decide which voltage is one or the other: these are the logic levels.
During the years, with the improvements of technology, the actual value of the
logic level assigned to the value
HIGH is diminished from 5V to 3.3V, to
2.5V to 1.8V and 1.5V.
It's usually indicated as TTL that means Transistor-Transistor Logic.
Connected system with different logic levels must use a logic level converter to
avoid damaging the device al lower voltage (also to avoid missing
LOW level device).
A buffer (like all logic gates) is an active device. It requires additional inputs to power the gate, and provide it voltage and current.
Digital Buffers can be used to isolate other gates or circuit stages from each other preventing the impedance of one circuit from affecting the impedance of another. A digital buffer can also be used to drive high current loads such as transistor switches because their output drive capability is generally much higher than their input signal requirements. In other words buffers can be used for power amplification of a digital signal as they have what is called a high “fan-out” capability.
- How to Build a Buffer Circuit with a Transistor a little overlong explanation
Open collector mode
In this mode a pin has two possible state: connected to
GND (probably via a transistor)
or open circuit.
An advantage is that more than one open-collector output can connect to a single line. If all outputs attached to the line are in the high-impedance state, the pull-up resistor will hold the wire in a high voltage (logic 1) state. If one or more device outputs are in the logic 0 (ground) state, they will sink current and pull the line voltage toward ground.
- Open Collector Outputs
- Page of wikipedia
- Introduction to Wired-OR Outputs and Open-Collector Circuits
- Choosing an Appropriate Pull-up/Pull-down Resistor for Open Drain Outputs
Slide with some history and ratio about packages
- DIP/DIL: Dual In Line, through hole component
- SOIC: Small Outline IC, surface mount component
- SOT: Small-outline transistor (wikipedia page)
- QFN: Quad Flat No leads
- QFP: Quad Flat Packages
- LQFP: Low-profile quad flat packages
- TQFP: Thin quad flat packages
- Sparkfun's tutorial
- TI's application note about packaging
It's an integrated circuit that allows to multiplex from one input, multiple outputs.
The shift term in its name refers to the fact that data is entered serially from a unique pin and the existing data is shifted by one place at times. Tipically they work with a minimum of three input pins:
SRCLK: the serial clock that decides (by its raising edge) when shift the register
SER: serial pin that decides what value must be inserted in the new register
RCLK: register clock that decides (by its raising edge) what value store in the new register
In practice you usually present the value with which you want to configure the output pins
alternating a raising edge with
SRCLK and using the
SER pin for each value; during
this operation the
RCLK must be tied
LOW to avoid the updating of the output pins,
only when you are happy with the values
RCLK must be tied