conn

Documentation

Overview

Package conn defines core interfaces for protocols and connections.

This package and its subpackages describe the base interfaces to connect the software with the real world. It doesn't contain any implementation but includes registries to enable the application to discover the available hardware.

Concepts

periph uses 3 layered concepts for interfacing:

Bus → Port → Conn

Not every subpackage expose all 3 concepts. In fact, most packages don't. For example, SPI doesn't expose Bus as the OSes generally only expose the Port, that is, a Chip Select (CS) line must be selected right upfront to get an handle. For I²C, there's no Port to configure, so selecting a "slave" address is sufficient to jump directly from a Bus to a Conn.

periph doesn't have yet a concept of star-like communication network, like an IP network.

Bus

A Bus is a multi-point communication channel where one "master" and multiple "slaves" communicate together. In the case of periph, the Bus handle is assumed to be the "master". The "master" generally initiates communications and selects the "slave" to talk to.

As the "master" selects a "slave" over a bus, a virtual Port is automatically created.

Examples include SPI, I²C and 1-wire. In each case, selecting a communication line (Chip Select (CS) line for SPI, address for I²C or 1-wire) converts the Bus into a Port.

Port

A port is a point-to-point communication channel that is yet to be initialized. It cannot be used for communication until it is connected and transformed into a Conn. Configuring a Port converts it into a Conn. Not all Port need configuration.

Conn

A Conn is a fully configured half or full duplex communication channel that is point-to-point, only between two devices. It is ready to use like any readable and/or writable pipe.

Subpackages

Most connection-type specific subpackages include subpackages:

→ XXXreg: registry as that is populated by the host drivers and that can be leveraged by applications.

→ XXXtest: fake implementation that can be leveraged when writing device driver unit test.

→ XXXsmoketest: smoke test that tests against real hardware to ensure the whole stack work correctly, including the OS supplied drivers.

Index

• type Conn
• type Duplex
  • func (i Duplex) String() string
• type Limits
• type Resource

Examples

• Conn

Types

type Conn

type Conn interface {
	// Tx does a single transaction.
	//
	// For full duplex protocols (generally SPI, UART), the two buffers must have
	// the same length as both reading and writing happen simultaneously.
	//
	// For half duplex protocols (I²C), there is no restriction as reading
	// happens after writing, and r can be nil.
	//
	// Query Limits.MaxTxSize() to know if there is a limit on the buffer size
	// per Tx() call.
	Tx(w, r []byte) error
	// Duplex returns the current duplex setting for this point-to-point
	// connection.
	//
	// It is expected to be either Half or Full unless the connection itself is
	// in an unknown state.
	Duplex() Duplex
}

Conn defines the interface for a connection on a point-to-point communication channel.

The connection can either be unidirectional (read-only, write-only) or bidirectional (read-write). It can either be half-duplex or full duplex.

This is the lowest common denominator for all point-to-point communication channels.

Implementation are expected but not required to also implement the following interfaces:

- fmt.Stringer which returns something meaningful to the user like "SPI0.1", "I2C1.76", "COM6", etc.

- io.Reader and io.Writer as a way to use io.Copy() for half duplex operation.

- io.Closer for the owner of the communication channel.

Example

// Get a connection from one of the registries, for example:
//   b, _ := spireg.Open("SPI0.0")
//   c, _ := b.DevParams(1000000, spi.Mode3, 8)
var c Conn

// Send a command over the connection without reading.
cmd := []byte("command")
if err := c.Tx(cmd, nil); err != nil {
	log.Fatal(err)
}

type Duplex

type Duplex int

Duplex declares whether communication can happen simultaneously both ways.

Some protocol can be either depending on configuration settings, like UART.

const (
	// DuplexUnknown is used when the duplex of a connection is yet to be known.
	//
	// Some protocol can be configured either as half-duplex or full-duplex and
	// the connection is not yet is a determinate state.
	DuplexUnknown Duplex = 0
	// Half means that communication can only occurs one way at a time.
	//
	// Examples include 1-wire and I²C.
	Half Duplex = 1
	// Full means that communication occurs simultaneously both ways in a
	// synchronized manner.
	//
	// Examples include SPI (except 3-wire variant).
	Full Duplex = 2
)

func (Duplex) String

func (i Duplex) String() string

type Limits

type Limits interface {
	// MaxTxSize returns the maximum allowed data size to be sent as a single
	// I/O.
	//
	// Returns 0 if undefined.
	MaxTxSize() int
}

Limits returns information about the connection's limits.

type Resource

type Resource interface {
	// Halt stops the resource.
	//
	// Unlike a Conn, a Resource may not be closable, On the other hand, a
	// resource can be halted. What halting entails depends on the resource
	// device but it should stop motion, sensing loop, light emission or PWM
	// output and go back into an inert state.
	Halt() error
}

Resource is a basic resource (like a gpio pin) or a device.

Implementers are expected to also implement fmt.Stringer. In this case, String() must return a name representing this resource in a descriptive way for the user.