Other Parts Discussed in Thread: MSP430FG4619, MSP430F6720
Hello,
Great forums, great chips! The MSP430 line of microcontrollers is quite amazing. But nothing is perfect, here's my problem:
I'm making a specialized timer and need a simple display using two multiplexed 7 segment, 6 digit (5 are desired, 4 - 1/2 grudgingly acceptable.) LCD glass displays (Lumex). The displays are driven directly (no external controller). Each display has one common line. I've built a prototype around a MSP430FG4619 and its LCD_A controller. I'm using the external reference input of the charge pump to generate sufficient voltage to provide good contrast with the two displays multiplexed and 1/2 bias. When the external input is at about 1.5 volts I get great contrast and everything is working nicely. The MSP430FG4619 is almost perfect except for the following.
1. I need the charge pump, and when using the charge pump the lower four segment lines cannot be used, and I want all 40 segment lines if possible. Otherwise I give up controlling colons and/or all segments of one digit. I can live with that for now.
2. The LCD displays have nice contrast but I worry that the output voltage from the charge pump (which approaches 6 volts when the external reference is > 1.5 volts!) is ultimately over stressing the MSP430 and is prone to fail (so far, it's been running for several months with no ill-effect).
It's not real clear to me from the data sheet what the in/out voltage limits of the charge pump are. Are they ultimately supposed to be limited to within a diode drop of the supply voltage? That can't be strictly true for the LCD segment lines/charge pump, else - why haven't I popped the FG4619 yet nor drawn excessive current? It's operating with only about 300 uA of total average current (3 volt supply) while driving 6 volt waveforms to the display segments. Pretty amazing.
3. The MSPFG4619 is too expensive.
Alternatively, I discovered the MSP430F6720 which is three times less expensive and has the more advanced LCD_C controller. It also has sufficient memory and other resources for my project. However, I cannot get the external charge pump reference input to work at all after setting all the control registers properly, and the data sheet is more clear about limitations of the voltage range of the charge pump, making me wonder if I can ever get sufficient output voltage to drive the LCD glass displays with good contrast. They do work with 3.4 volts out with the charge pump referenced to internal Vdcc, but the contrast is much poorer than what I got from the MSP430FG4619.
So, either I figure out how to use the external reference with the charge pump to generate more voltage, if it's possible with this chip
OR -
I find a low-voltage LCD glass that has good contrast with 3.3 volts 1/2 bias. It seems that the LCD glass manufacturers do not specify the operating voltage range of their bare 7-segment displays very well if at all - they might say 3-5 volts, but with no mention of contrast ratio, frequency, etc.
OR,
Find an alternative microcontroller with a better integrated LCD controller.
I'm loath to use an external controller with its added expense and power, and I prefer the look of 7 segment displays to the dot-matrix character modules anyway.
Thanks for reading. Any advice?
Dan