Driving a character LCD using PIC24 Enhanced Parallel Master Port

E strobe train

E strobe train

Traditionally, Toshiba HD44780-compatible alphanumeric LCD displays are driven by bit-banging bus signals combined with long delays between sending commands and data. In many cases this method is good enough. There are other cases as well where extra CPU cycles are not available and more economical method of driving a display is needed. I’m currently working on a design involving very fast USB exchanges combined with occasional LCD output and developed a solution which works very well for me. I’m posting it with hope that my fellow developers will find it useful.

HD44780 displays have been around for a long time. The internet provides plenty of posts about them, code samples and even a Wikipedia article. My favorite introductory text on the topic is Dincer Aydin’s LCD Info page.

PIC24 16-bit microcontrollers from Microchip have been around for some time as well. They are cheap and powerful and the Microchip C30 compiler (free version available) is quite good. They are not as popular as their 8-bit counterparts from Microchip and Atmel therefore good PIC24 resources are scarce. One nice introductory text on the topic can be found at Engscope.

Since I’m trying to minimize CPU time spent driving the LCD let’s first talk about timing in general. When developing for HD44780 we need to deal with 3 different times. First is the timing of the display part – the screen we see. LCD glass is very slow. When we attempt to update the screen faster than say twice a second the symbols become blurry and pale. The fastest display in my collection still looks OK when updated at 4Hz rate (250ms), while most others are twice as slow.

On the other hand, display data bus timing is many times faster. In order to write to the display we first need to set RS, RW and data lines, wait a little, then assert E line, wait some more and then de-assert it. If we are reading from the display we will also need to wait a little more after de-asserting E before we can read the data on the bus. Total bus cycle length is ~2.5us, which is 200 000 times less than the update rate of typical LCD glass. This time is pretty short but the MCU is still faster – a PIC24F clocked at 32MHz has an instruction cycle of 62.5ns and in 2.5us it will be able to execute 40 instructions. Therefore, no matter how simple it looks, it is preferable not to bit-bang the bus.

The third timing we need to deal with is command execution time. All but two LCD commands have stated execution time of 40us. Two slow commands – Clear and Home require 1.64ms to finish. Those are datasheet numbers, in reality the fast command on a modern display may finish in as low as 10us and slow commands on an old display can take as much as 3.5ms, depending on the age and the particular “HD44780-compatible” controller used. It is about 100 times faster than the glass.

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Adding a display to a digital scale using Arduino and USB Host shield

Arduino reading digital scale

Arduino reading digital scale

I am the proud owner of Model 510 5lb digital scale. It is a nice little scale which works very well (much better than service itself) while attached to my workstation. The scale doesn’t have a display making any kind of standalone use difficult. However, since the scale is a USB HID device reading data from it should be as easy as from a joystick and Arduino board should be adequate to provide a display function for it. To test this theory I made a simple setup consisting of Arduino UNO, USB Host shield and HD44780-compatible LCD display. I also wrote a small sketch which polls the scale and outputs the weight. The secondary objective of this project was to demonstrate LCD support in USB Host shield library.

For this project I used the following:

  1. An Arduino board. Standard size board, such as UNO, Duemilanove or Leonardo, will work
  2. USB Host Shield
  3. Toshiba HD44780-compatible LCD display, in 16×1 or 16×2 configuration. If you’re planning to use this sketch for something else, like data logging, the display is optional – all output from the scale is repeated to the serial port
  4. 5lb digital scale. Scales are standard HID devices with usage table 0x8d, therefore, scales from other brands may work as well with no or minimal modifications to the code
  5. USB Host library

The example code is also hosted at github, as well as in ‘examples’ section of the library under ‘HID’. It has been tested with Arduino IDE version 1.0.5.

In this project, the LCD is connected to the shield’s GPOUT pins, as documented in max_LCD.h header file. In addition to data lines, 5V and ground must also be connected to the shield’s 5V and GND terminals; the RW pin must be grounded – I do it on the LCD itself. In order to see the characters, the display must be biased – a 5K-10K pot with wiper on Vo and other two pins on 5V and ground will provide contrast adjustment.

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CueCat meets Arduino, works with HID boot code

CueCat meets Arduino

CueCat meets Arduino

Soon after posting Arduino barcode scanner article I started receiving questions about CueCat. Many of these cat-shaped devices were distributed free of charge in the US at the end of the dot com craze and even though company which developed and distributed them went out of business long time ago, the USB CueCats are still available for very reasonable price. Declawed CueCat with USB interface can be purchased for as little as $13 total in the US and non-modified ones for even less; at the same time, typical no-name handheld barcode scanner goes for around $25 on eBay and will be shipped from China.

I tested “declawed” USB CueCat with my code – it initializes as HID boot keyboard and works very well. No modifications are needed to the sketch from the previous article – just plug in the device and start scanning. CueCat reads many different barcode systems, including UPC and ISBN. Since CueCat is in constant scanning mode (no button needs to be pressed to initiate a scan), it can be used for applications like automated inventory control or as a part of a motion feedback circuit in CNC/robotics project.

One last advantage of CueCat I’d like to point out is low power consumption. As can be seen on a title picture, an Arduino, USB Host Shield, CueCat and HD44780-compatible LCD display can be run from a small LiPo boosted to 5V by Mintyboost from Adafruit Industries with its output connected to Arduino USB port.

CueCat is compact and inexpensive barcode scanner still available for sale despite being discontinued many years ago. It is implemented as USB HID boot device and supported by USB Host library. If you were living in the US in 2000, chances are you already have one or even several of these cat-shaped devices laying around – try it with my code and let me know if you have any issues.


Connecting barcode scanner to Arduino using USB Host Shield

Scanning barcodes using Arduino and USB Host Shield

Scanning barcodes using Arduino and USB Host Shield

An addition of Human Input Device Class support to USB Host Shield library 2.0, announced several days ago allows using powerful and inexpensive input devices with USB interface in Arduino projects. Sample sketches demonstrating sending and receiving data to one of the most useful HID device types – boot keyboard/mouse, has been released along with the library. The beauty of boot protocol lies in the simplicity of device report – a data packet containing information about button presses and mouse movements. However, samples were designed to demonstrate all features of the class and because of that, they are somewhat heavy. In real-life applications, it is often not necessary to implement each and every virtual function – only what is needed. In today’s article I will show practical application of HID boot device building a simple gadget.

Originally, HID boot protocol was meant to be used with keyboards and mice. When USB became popular, other keyboard-emulating devices, such as barcode scanners and magnetic card readers have been migrated from PS/2 standard to USB while keeping their keyboard-emulating property. As a result, many modern “not-so-human” input devices behave exactly like a keyboard including boot protocol support. A gadget that I demonstrate today is portable autonomous barcode scanner built using Arduino board, USB Host shield, handheld USB barcode scanner and LCD display (see title picture). The operation is simple – when handheld scanner button is pressed, it scans the barcode and sends it to Arduino symbol by symbol. Arduino then outputs these symbols on LCD display. LCD is erased before outputting each new barcode by tracking time between arrival of two consecutive symbols. To keep the code simple, I intentionally did not implement any data processing, however, since Arduino sketch for the gadget compiles in just a little over 14K, there is plenty of memory space left for expansion.

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Arduino-based controller for Canon EOS cameras

Arduino Camera Controller

Arduino Camera Controller

Today, I’m writing about two more pieces of digital camera control firmware that have being posted to PTP gihHub repository. Alex Gluschenko, the author of PTP library for Arduino, developed two sketches, one called EOSRemote and the other EOSCamController to demonstrate PTP library capabilities. The code allows requesting camera settings, such as shutter speed, aperture, ISO, etc., change them, as well as take shots. It supports Canon EOS cameras and was tested on EOS 400D, 450D, and 7D; other cameras with similar command set ( see my collection of PTP device info dumps ) may work as well.

This is how the code works: when connection is established, camera sends back an initial packet with all its current settings along with a list of all possible values for each setting. Possible values depend on a camera model as well as lens that are mounted. For example, some cameras may have exposure compensation range from -2 to +2, others from -5 to +5; some lens have max.aperture 1.4, others – 3.5, an so on. The list of values received from the camera is placed in built-in EEPROM of Arduino microcontroller. After that, values are used in setting (called “property” in PTP lingo) change commands sent to the camera. When camera mode, such as Av, Tv, or lens is changed, camera sends initial packet again. The Arduino code tracks changes and updates the list stored in EEPROM.

Hardware requirements for both controllers are pretty standard. The EOSRemote sketch uses Arduino serial port facility for I/O, and the only hardware necessary is Arduino Duemilanove, Uno, or compatible clone, as well as USB Host Shield. User interface is designed with simple terminal emulator program in mind. It can be used with a PC connected to Arduino directly or over some distance utilizing a pair of serial to RF converters, such as ever-popular Xbee. The following screenshot shows top-level menu with camera connected and recognized.

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Interfacing LCD via SPI.

HD44780 LCD display

HD44780 LCD display


As time goes by, microcontrollers become more powerful, cheaper, and smaller. A typical micro of the past could have had 40 pins and no internal memory. On the contrary, modern J-series PICs are made with 96K program memory and 28 pins. We can drive a lot of peripherals with that amount of memory, however we are getting short on pins.

In this article I will show how to drive a parallel interface peripheral serially. A HD44780-compatible LCD module is good candidate – it is popular, inexpensive, and slow, so you won’t be losing any speed while converting parallel to serial. And you could even save some money using a micro with fewer pins.

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