At 05:30 PM 6/6/03 -0400, Chris Loiacono wrote: >First, an open admmission - I'm still learning to apply LCD's and don't know >enough yet. >In working with static 7-segment units, I have read that they can be run by >toggling the commons at a fairly low frequency as with GPIO while >controlling the segment pins with steady output. I have also read that the >devices will be damaged if there is any substantial DC offset as seen by the >segments. > >A paper on the eio.com site suggests that CMOS XOR gates be used to shift >the phase of the applied control pulse to the segment, and it skips right to >the understanding that by doing so, the seg will see no DC offset. I can't >seem to cross the bridge to that point. Small static drive LCDs are easy to drive. I generally use 4- 74hc595 shift registers for the common 40 pin displays (3.5 digits) often seen in cheap DVMs. As you have seen, you have to drive them with an AC waveform. I usually run my units at about 40 Hz but it is not critical. Here's how: connect the LCD segments to the outputs of the shift registers. Also connect the LCD common to a SR output. Arrange your code so that you shift a complete packet out to the shift registers at double the desired frequency. You want a 50% duty cycle, so take some care that the time between each packet is about the same. In order to have an AC signal, you have to invert the data each time you send a packet. In other words, alternate packets send true data, then inverted data. The LCD common follows the state of the toggle flag. And that is all it takes! One of my projects drives 8 digits (2- 4 digit displays) in about 150 code spaces and uses TMR0 to keep track of the packet rate (12.5 ms period). Its also small and very inexpensive - those 'hc595 shift registers cost pennies. dwayne -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 19 years of Engineering Innovation (1984 - 2003) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics