Small LCD modules are the display of choice for many front-panel requirements. Standard 14-pin modules are available from multiple vendors, and for about $10 they offer one to four display lines, eight to 40 characters per line, a simple parallel interface and an ASCII-character input. The down side in many cases is their 5-V operation. Designers of 3-V systems must either invent a custom display or add power supplies and level shifters to accommodate the standard 5-V display.
This simple circuit allows common LCDs to run on supply voltages as low as 2.7 V. A charge pump that can supply up to 50 mA at 5 V (Id) boosts the supply voltage from 3 V to a regulated 5 V. To avoid power-supply sequencing considerations, the display and the contrast-control voltage are both powered from the boosted 5-V rail. The CMOS LCD module has TTL-compatible logic-level inputs (0.8 V low and 2.4 V high), so the 3-V microprocessor should be a CMOS device whose logic outputs (near 3 V high and 0 V low) enhance the noise margin.
A few limitations should be noted. First, the display module has a read/write interface, but to avoid level-shifting 5-V signals from the display, this circuit operates the display in write-only mode by grounding its /WR\ pin. Write-only operation has two consequences. Because the display-ready flag cannot be read, the designer must rely on specified maximum timing values, which tend to be conservative. The result, typically, is a 50 percent slowdown in display operations. Worst-case timing remains the same, however, and even the worst-case timing is fast.
As a second consequence of write-only operation, the display module's excess memory, if any, cannot be used by the system as scratchpad memory. On the other hand, this limitation is not severe-the display interface is slow, compared with SRAM-and only a few dozen bytes of memory are available.
As a second general but minor limitation in this circuit, the LCD supply current is higher at 3 V than at 5 V. That behavior, in which both MOSFETs of a CMOS pair turn on slightly for less than full-rail conditions, is characteristic of CMOS devices. The current is about 3 mA with all lines at 3 V and 400 microamps with all lines low. In most applications that higher current flows only a few times per second, for only a few tens of microseconds at a time, and only when the high levels are applied. In any case, you can minimize supply current by writing all microP lines low following each write operation.