HP-42S "reinterpretation"

[robert rozee]

(05-04-2015 03:49 PM)pito Wrote:  HD44780 based LCDs require 5V Vcc.. Also the display is not graphical one, but 2x16 chars.. An SPI color 1.8-2.4inch tft LCD will do better, power consumption <<1mA with backlit off. Backlit needs a few mA (the same with HD44780). (Btw HD44780 LCDs do not require negative LCD bias..)
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All the components - LCD, LCD backlit, BT, sdcard, 24LCs would need a high-side pmos switch (thus you could switch them off to save power).

the HD44780 work-alike chips used on most modern LCD modules will run quite happily on 3v, but for the LCD screen to be visible there still needs to be around 5v between Vcc and the contrast pin - hence the provision of a negative supply. a fellow engineer came up with that part of the solution, adapted from one of his other projects. using a PWM output on the processor to drive the charge pump allows for electronic contrast adjustment.

other display choices (colour, back lighting, etc) will suck battery power. and the HP-42S/Free-42 firmware has no support for colour or graphics, splicing either in would provide a major roadblock on any project timeline.

the PIC32MX170 is capable of sleeping on 20uA, so no need for any battery switching. compared to the 3000mA.h capacity of AA cells, 20uA is insignificant. and between keypresses the processor can sleep. running at 40MHz the current consumption is around 25mA, giving 120 hours of continuous running a program.

choice of components is also constrained by the requirement for through-hole components throughout. SMD parts are strictly excluded.


cheers,
rob :-)