LCD to USB interface

Posted on May 2, 2012

LCD2USB is a open source/open hardware project. The goal of LCD2USB is to connect HD44780 based text LCD displays to various PCs via USB. LCD2USB was meant to be cheap and to be made of easily available parts. It is therefore based on the Atmel AVR Mega8 CPU and does not require any difficult to obtain parts like separate USB controllers and the like. The total cost (without display and pcb) are about 5 to 10 Euros. LCD2USB currently comes with a simple demo application that works under Linux, MacOS X and Windows.

LCD to USB interface
Click here to download the full size of the above Circuit.

LCD2USB is currently supported by lcd4linux (LCD2USB support is built-in), LCD Smartie (requires a seperate driver), and LCDProc (LCD2USB support is built-in). The hardware of the LCD2USB interface consists of the Atmel AVR Mega8 CPU, a cheap and easy to obtain microcontroller with 8 KBytes flash (of which ~3k are used in this application) and 2 KBytes RAM. The processor is surrounded by few parts, mainly connectors to interface to the PC and the LCD. A power LED (LED1) indicates that the system is powered via USB. The system clock is derived from a 12Mhz crystal. This frequency is necessary due to the fact that the software USB implementation requires a precise timing with respect to the USB. The USB interface of the LCD2USB interface is based on a pure software implementation and uses three pins of the AVR (PC0, PC1 and PD2). This software implementation supports low speed USB only which is signalled to the PC by resistor R1. The current version 1.1 of the LCD2USB operates the USB data lines at 3.6V which complies to the USB spec and increases compatibility over version 1.0. Special care has to be taken with displays with backlighting. The LCD2USB has been designed to draw at most 75mA which is fine for typical LCDs with LED backlighting. This value is also reported to the host PC via the USB configuration. The LCD2USB interface hardware is able to supply up to 100mA via its software adjustable backlight control. This is...

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