Dragging the Psion 5MX into the 21 century using the Octavo Systems OSD3358 SiP
So I've been hammering away at this for a couple weeks, need to start sharing publicly and posting progress to some form of blog to keep my sanity.
I've been intriegued by Psion 5MX modernization projects, and whilst I love the idea of sticking a Raspberry Pi of some sort inside, it's not so easy to integrate with existing hardware (keyboard / antique LCD).
So, after recently playing with the PocketBeagle and learning that the board is a minimal breakout of the OSD3358, I'm really liking the idea of trying to use this system in package inside the 5MX. It's not the most powerful, nor does it have the most RAM, but it does seem to fit with the original intent of the Psion computers: a low power system that may be able to run for some time on a battery.
I've imported the keyboard controller from RasmusB's Psioπ, and am looking to add USB Type C to allow easy charging and OTG peripherals, an SDIO Wifi/BT chipset, and cut out a chunk of the original mainboard to allow a lithium cell to sit inside.
My current headache: the LCD. There is no datasheet I can find, I even called Sharp Displays, nada. I know the LCD requires bias voltages of [-14.8v, 18v] to [-19.4v, 22.8v], but trying to find a boost regulator solution that is adjustable in voltage to allow setting of contrast (that range is from min to max contrast) is a pain in the butt.
I've considered voltage multipliers, I can build one, perhaps with a GreenPAK SLG64826 as the controller for both negative and positive, but the multiplication required is gonna waste a lot of space with capacitors and diodes. That was a fun side quest, SPICE simulation followed by breadboarding with the GreenPAK dev kit, but probably less feasible.
Back to switching regulators (as used by Psion, the oldskool way with separate ICs [comparators, etc]), there are solutions for positive and inverted that are less adjustable (perhaps a DigiPOT or DAC to adjust contrast?), or the MAX1620 x2, awesome, adjustable, I2C would clash, but I can just use the up/down control variant.
What to do...