Chips come in a few common packages, most of which we'll ignore because they require soldering or don't easily fit on a breadboard or protoboard.
- Leads, or pins, are the metal slivers sticking out of the chips.
- A pinout is an image showing the order of the pins, often with a brief or incomplete listing of its functionality or identifier(s).
- Pitch, or pin spacing, is how far apart the pins are, measured between the centers of two, adjacent pins.
- Through-hole is when the pins point down, perpendicular to the chip, long enough to pass completely through a board for soldering. On brief part descriptions (such as the bags parts come in from retailers), this is often abreviated T/H.
- Surface mount is when the pins stick out to the sides (or there are no pins), meant for soldering directly to the top of the circuit board.
This is by no means a complete list. Avoiding soldering gives very few options. Full list here.
- DIP (dual inline package): these are what we'll use. Pins are situated in two, parallel rows. Pin spacing is extremely standard and is ideal for breadboards and protoboards. DIPs are often prefixed with a material; a special case is plastic, commonly abreviated PDIP.
- TO-# (# can be one to three digits): packages for small pin count components such as transistors and voltage regulators. These are often through-hole packages, but may be surface mount.
- QFP (quad flat package): these contain pins on all four sides, requiring soldering or breakout boards. Pin spacing comes in multiple flavors, usually 1mm or 0.4mm (the latter refered to as fine-pitched). Common, related packages are thin QFP (TQFP) or low-profile QFP (LQFP) for those with extremely low profiles. As with DIPs, these can also be prefixed with materials.
- SOIC (small outline integrated circuit): essentially, surface mount DIPs, with many variants for pitch/etc. These often, or always, share the same, exact pinout as their DIP counterparts.
How to Read Pinouts
Pinouts contain pin numbers and descriptions or identifiers for each pin. It's necessary to delve further into the documentation for full descriptions and capabilities, as well as how to use each.
Pin numbers are consistent, starting from the "top-left" and continuing counter-clockwise around the package. Top-left depends on the package but is denoted by a small circle in the corner closest and, often, in DIPs, by a half-circle carved out of one side (the "top").
Power and Grounds
- Pins for power are describe in multiple ways, most commonly VCC, V+, VDD, VS+ and VBAT. Different notations are often synonymous but may denote different functions or power supplies for particular subsystems.
- Pins for ground, or "negative supply voltage," also have multiple notations, most commonly GND, VSS, VEE, V-, and VS-.
Side note: the letter 'A' prefixing voltage supplies or grounds stands for analog. They should (usually) be connected and are used for analog-to-digital convertors and such so should be isolated from the main power supply to decrease noise (but should be the same voltages). When in doubt, RTFM.
Other Common Pin/Function Descriptors
These may be the entire description or just part of one, but these will give you a general idea of how to translate them. Also not a complete list, as pin naming isn't standardized.
- NCs are not connected. Many chips come in packages with more pins than are required, especially quad packages. You can safely ignore these.
- PIO#_#/P##/GPIO: general I/O pin. The two numbers may be ommitted or may be letters. These can simply be set high or low in code (high and low refer to the supply voltage and ground).
- RESET: often inverted, triggering this pin will restart the chip.
- RTC: real-time clock.
- XIN/XOUT/XTAL: XTALs are crystal oscillators
- ADC: analog-to-digital converter.
- INT: interrupt pin. External changes to these pins can trigger the chip to run specific code.
- TWI/I2C: two wire interface.
- SPI: serial peripheral interface
- ISP: in-system programming
- UART/USART: universal (synchronous)/asynchronous receiver/transmitter
- TX/RX: transmitter and receiver pins, usually via UART
- USI: universal serial interface (usually, multiple communication protocols)
- PWM: pulse-width modulation
- WDT/WDR: watch dog timer and watch dog timer reset
- CL/CK/CLK : clock
- SD: serial data
- SCK/MOSI/MISO/SS: used for programming the chip
Side note: while reading microcontroller datasheets, pins are a single lead and ports are collections of pins (usually 8).
Another quick side note: flat lines over a description denote that the pin is inverted, meaning that grounding it is a true, or "on", state.