hd44780: Restructure to implement conn/display/TextDisplay. Refine to use GPIO.Group (#91)

* Add support for Adafruit I2C/SPI Backpack
* Add support for PCF857x Backpacks
* Add support for conn/gpio/Group
* Implement conn/display/TextDisplay
* Add examples
* Cleanups
* Change backlight implementation

Author: gsexton

go.mod

@@ -12,7 +12,7 @@ require (
 	github.com/mattn/go-colorable v0.1.13
 	golang.org/x/image v0.23.0
 	periph.io/x/conn/v3 v3.7.2
-	periph.io/x/host/v3 v3.8.3
+	periph.io/x/host/v3 v3.8.4
 )
 
 require (

go.sum

@@ -17,5 +17,5 @@ golang.org/x/sys v0.29.0 h1:TPYlXGxvx1MGTn2GiZDhnjPA9wZzZeGKHHmKhHYvgaU=
 golang.org/x/sys v0.29.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 periph.io/x/conn/v3 v3.7.2 h1:qt9dE6XGP5ljbFnCKRJ9OOCoiOyBGlw7JZgoi72zZ1s=
 periph.io/x/conn/v3 v3.7.2/go.mod h1:Ao0b4sFRo4QOx6c1tROJU1fLJN1hUIYggjOrkIVnpGg=
-periph.io/x/host/v3 v3.8.3 h1:v90ozCFDWgEyfNElZ+JnOvq0jAdW0vmgjCUy8dYXDds=
-periph.io/x/host/v3 v3.8.3/go.mod h1:uKrIpfXjELwHkwGBNe6aos//XiQ/3uxDa1P2BmLV6Ok=
+periph.io/x/host/v3 v3.8.4 h1:QNleTythDd0k6Chu0n+ISrJFlf3LFig9oNbtOIkxoCc=
+periph.io/x/host/v3 v3.8.4/go.mod h1:hPq8dISZIc+UNfWoRj+bPH3XEBQqJPdFdx218W92mdc=

hd44780/README.md

@@ -0,0 +1,60 @@
+# Hitachi HD44780 Package
+
+## Overview
+
+The Hitachi HD44780 is an LCD driver chip. It's used in a variety of text LCD 
+displays. The datasheet is available here:
+
+https://www.sparkfun.com/datasheets/LCD/HD44780.pdf
+
+Generally, there are three kinds of displays that use this chip.
+
+1. A raw display. This is a board with a row of pin connectors at the top. It's
+made for interfacing directly to GPIO pins. This is the most complicated method
+of using the LCD because you have to wire a minimum of 6 GPIO pins (4 data, 1 
+reset, 1 enable) and 2 power pins to make it work.
+
+There are also complex initialization routines that have to be performed. This
+is further complicated by having specific initialization calls depending upon
+whether the device is connected to 4 data lines, or 8 data lines.
+
+2. A backpack display. With this type of LCD display, an I2C, serial, or SPI
+interface is provided. This type of display is easier because there are fewer
+pins, but the complex intialization remains. Examples of backpack interfaces
+include The Adafruit I2C/SPI backpack (MCP23008/74HC595D), the generic 
+LCDXXXX/PCF8547T backpack, etc.
+
+3. The third and final variety is the "Intelligent" display. These displays have
+a micro-controller that is connected to the LCD chip. Typically these intelligent
+displays support multiple I/O methods and the micro-controller handles the
+LCD initialization/communication.
+
+Examples of this kind of display would be the SparkFun SerLCD display, the
+MatrixOrbital LK2047T, the AdaFruit USB+Serial Backpack etc.
+
+## Interfacing Notes
+
+The driver package is designed to use the gpio.Group interface. This allows
+the LCD driver to be agnostic about the physical connection between the display 
+and the host device. Any host/expander that supports the 
+periph.io/x/conn/v3/gpio.Group interface can be used to easily drive the LCD 
+display.
+
+## Hardware Notes
+
+DO NOT attempt to source VCC for the unit backlight, or sink VCC to ground.
+The backlight draws ~250ma of current which exceeds the current capability
+of GPIO pins. It will permanently damage your device. If you would like to 
+control the backlight, connect a GPIO pin through a 1K Ohm resistor to a 
+transistor (2N2222 or equivalent).
+
+## Troubleshooting
+
+If nothing displays at all, check the contrast. Adjust the contrast control
+until the 5x7 dot grid on the display is visible.
+
+If the first row contains blocks of dots, and the other rows are blank, then
+the initialization of the device failed. Check IO pins are connected properly.
+
+If the text is garbled, verify the gpio.Group is configured and the IO Pins
+are connected to the right pins of the LCD display.

hd44780/af_i2c_backpack.go

@@ -0,0 +1,62 @@
+// Copyright 2025 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package hd44780
+
+import (
+	"periph.io/x/conn/v3/gpio"
+	"periph.io/x/conn/v3/i2c"
+	"periph.io/x/conn/v3/spi"
+	"periph.io/x/devices/v3/mcp23xxx"
+	"periph.io/x/devices/v3/nxp74hc595"
+)
+
+const (
+	// Name is the LCD pin name, and the integer value is the GPIO
+	// number (not physical) of the MCP23008 I2C GPIO Expander.
+	d4           = 3
+	d5           = 4
+	d6           = 5
+	d7           = 6
+	rsPin        = 1
+	enablePin    = 2
+	backlightPin = 7
+)
+
+// This function returns a display configured to use the Adafruit I2C/SPI LCD Backpack.
+//
+// # Product Information
+//
+// https://www.adafruit.com/product/292
+//
+// The I2C side of this backpack uses an MCP23008 I/O expander. This function
+// creates an MCP23008 device with the required pin configuration. To use this,
+// get an I2C bus, and call this function with the bus, i2c address, number of
+// rows, and columns.
+func NewAdafruitI2CBackpack(bus i2c.Bus, address uint16, rows, cols int) (*HD44780, error) {
+	mcp, err := mcp23xxx.NewI2C(bus, mcp23xxx.MCP23008, address)
+	if err != nil {
+		return nil, err
+	}
+	gr := *mcp.Group(0, []int{d4, d5, d6, d7, rsPin, enablePin, backlightPin})
+	reset, _ := gr.ByOffset(4).(gpio.PinOut)
+	enable, _ := gr.ByOffset(5).(gpio.PinOut)
+	bl := gr.ByOffset(6).(gpio.PinOut)
+	return NewHD44780(gr, reset, enable, NewBacklight(bl), rows, cols)
+}
+
+// This function returns a display configured to use the SPI side of the Adafruit
+// I2c/SPI backpack. The SPI side uses a 74HC595 Serial->Parallel shift register.
+func NewAdafruitSPIBackpack(conn spi.Conn, rows, cols int) (*HD44780, error) {
+	chip, err := nxp74hc595.New(conn)
+	if err != nil {
+		return nil, err
+	}
+	// The SPI side has the same pins but in reverse order from the I2C side.
+	gr, _ := chip.Group(d7, d6, d5, d4)
+	rs := chip.Pins[rsPin]
+	e := chip.Pins[enablePin]
+	bl := chip.Pins[backlightPin]
+	return NewHD44780(gr, rs, e, NewBacklight(bl), rows, cols)
+}

hd44780/backlight.go

@@ -0,0 +1,40 @@
+// Copyright 2025 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package hd44780
+
+import (
+	"fmt"
+
+	"periph.io/x/conn/v3/display"
+	"periph.io/x/conn/v3/gpio"
+)
+
+// A monochrome backlight  Implements display.Backlight It uses a single
+// GPIO Pin to turn the backlight on or off.
+type GPIOMonoBacklight struct {
+	blPin gpio.PinOut
+}
+
+// Given a GPIO pin that turns the backlight on/off, construct a monobacklight
+// to use with HD44780.
+func NewBacklight(blPin gpio.PinOut) *GPIOMonoBacklight {
+	return &GPIOMonoBacklight{blPin: blPin}
+}
+
+// Turn the display backlight on or off.
+func (bl *GPIOMonoBacklight) Backlight(intensity display.Intensity) (err error) {
+	if intensity == 0 {
+		err = bl.blPin.Out(gpio.Low)
+	} else {
+		err = bl.blPin.Out(gpio.High)
+	}
+	return err
+}
+
+func (bl *GPIOMonoBacklight) String() string {
+	return fmt.Sprintf("%#v", bl)
+}
+
+var _ display.DisplayBacklight = &GPIOMonoBacklight{}