One wire brings power & data to LCD module
Embedded systems frequently use HD44780-type LCD displays as it is considered the most popular alphanumeric display controller. The interface comprises at least 14 pins: eight for data, three for control (EN, WR, RS), two for power supply (Vdd, Vss), and one for contrast (Vre). Configured in 8-bit mode, it requires at least 10 I/O lines (D0..D7, EN, RS). Configured in 4-bit mode, it requires at least six I/O lines (D4..D7, EN, RS). This last case seems usable when using an 8-pin PICmicro. However, 8-pin PICmicros have one pin as an input-only pin.
Many solutions for expanding I/O pins have been described in EDN Design Ideas (references 1 to 3). In a pin-limited external embedded system, if an additionnal LCD display becomes necessary, instead of changing the microcontroller with more I/O lines, it is often better (in terms of cost, and to make development easier and faster) to use this LCD module with a dedicated 8-pin PICmicro and consider it as an intelligent display.
The following design will be useful for any external pin-limited embedded system which needs to be interfaced to an HD44780 compatible display. Communication is via a one-wire serial link, using a simplified asynchronous protocol (8N1 at 2400bps). It uses a very cheap 8-bit microcontroller, the PIC12F508, but any other member of the baseline PIC12F family is suitable; the program code takes less than 256 words.
The serial link is used as a command/data line (for GP3 pin) and power line (for Vdd pin, through D1 and C1). The filter R2-C2 splits the GP5 pin to two I/O lines. If GP5 outputs a pulse greater than the time constant R2*C2 = 56µs, then input RS receives a high level, and data is written into the HD44780 display at the falling edge of the GP5 pulse.
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