diff -r 2f87039dd0b5 -r 5ce5f3046c0b content/numworks-lcd.rst --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/content/numworks-lcd.rst Wed Jul 12 21:35:29 2023 +0200 @@ -0,0 +1,186 @@ +========================== + Numwork N0110 calculator +========================== + +:Author: David Douard +:Category: Electronics +:Tags: Numworks, N0110, repair, calculator + + +My daughter's `Numwork `_ N0110 graphing calculator +got fried when plugged in a cheap USB PSU. Black screen, heating up killing the +battery in a matter of a few tens of minutes. + +The design being originally Open Source (hardware and firmware), it makes sense +to try to fix it instead of just buying a new one (it is not a cheap +calculator). + +The schematic is pretty simple: built around a `STM32F730V8T6 `_, there are +only a few components surrounding the MCU: + +- an LCD (280x320, 2,8", ST7789V driver) driven using the 16 bits 8080 + interface, taking advantage of ST's `FSMC `_ controller, + +- a 64MB flash (`AT25SF641 `_) to store apps and user data, + +- LiPo charger (`RT9526AGE `_), + +- `RT9078 `_ 2.8V voltage regulator to poser the MCU from the LiPo battery, + +- LCD backlight DC-DC converter (`RT9365GQW `_) + +- 3 LEDs plus a number of passives, keypad, reset button, + +- LiPo battery, + +- USB port surge protection diodes (`USBLC6-2SC6 `_) + + +The list of fried devices was pretty much every active component: + +- MCU, + +- LCD, + +- 2.8v regulator, + +- USB protection diodes. + + +Due to the global chip shortage, I had to gamble on Aliexpress to find a +STM32F730. I bought one from `there `_. Against all odds, the delivered chip +looks genuine and is working properly. + +So I replaced the MCU and the 2.8v regulator. At this point, I wasn't sure +about the status of the LCD. I did not know the USB protection diodes was dead, +so uploading a firmware was quite a challenge; USB based DFU was not working +(obvioulsy), but it was not obvious to understand why; I wasn't sure if a blank +STM32 should enumerate ok as USB device, or if bootstrping a firmware was +required before. + +I also did not want to upload Numworks' latest official firmware nor using +their WebDFU tools since they changes their licensing policy and moved to a +closed source model, official firmwares since version 16.3 now lock the +bootloader to prevent custom or open source firmware from being installed. +A complete explanation of this (in French) is `provided here `_ + +So I started looking at the alternative firmware projects for the Numworks: +`Omega `_, `Phi +`_, `Khi +`_ and `Upsilon +`_ (!) Plus starting +from a blank STM32 instead of an upgrade from an existing installation added a +bit of confusion. + + +After some time I got convinced I had successfully uploaded a firmware, but +noting showed up on the LCD: diagnostic of a dead LCD as well... + +The tricky part is that the LCD used in the calculator is pretty common on +principle: standard size, usual LCD controller. The very one used in the +calculator seems specificly made for the Numworks: specific ribbon cable and +specific pinout. Could not find a compatible device on usual sources. + +To validate I did indeed have a successfully replaced the MCU and uploaded a +working firmware, I did a nasty rastnest experiment with a ST7789V LCD module I +had around. + +The result was pretty ugly + +.. image:: {static}/images/numworks/rastnest.jpg + :alt: The test setup to connect a ST7789V based LCD module to the Numworks, + using a breadboard to interconnect the 2 interfaces. + :class: image-process-large-photo + +but I had *some* results: + +.. image:: {static}/images/numworks/partial.jpg + :alt: The LCD module shows half a screen with some content mixed with garbage. + :class: image-process-large-photo + + +First I was thinking the issue could be related to the speed of the +communications between the MCU and the LCD: the signal path was far from +optimal, with a ribbon cable, a breakout board, a bus of 10cm jump wires, a +breadboard, another bunch of 10cl jump wires, yet another bunch of jump wires, +another breakboard, and at last, the ribbon cable of the LCD module... So I +spent some time hacking the Omega source code to reduce the transmission speed; +but that did not help. + +To figure out what was wrong I ended up using my DSLogic logic analyzer. The +culprit was a pair of swapped jumpers (strangely there was only one pair of +messed connections). Since it was a pair of data lines above D7, it was +partially working. So the messed half display. + +With the 2 wires back in order, result was much better... + +.. image:: {static}/images/numworks/success.jpg + :alt: The LCD module shows the Numworks main screen... but flipped + :class: image-process-large-photo + +For some reason, this display showed the image reversed. Anyway, I was +confident enough to spend a bit more money buying a replacemement LCD module. + +Choosing a suitable replacement was not an easy task: + +- It needs to use the ST7789V controller. + +- It needs to be the proper size; seems easy, it's a standard 2.8" module... + well no, most of these modules are 66.95x47.80mm while the module in the + calculator is 66x50mm. Also the thickness must be about 2.1 or 2.2mm max. + +- It needs to have the proper ST7789V signals available, 16bits 8080 mode; + especially the Tearing Effect (TE) signal is used by the firmware to optimize + display refresh timing. + +- It needs to be an "12 o'clock" IPS LCD module (not a TN one) so the viewing + angle is adapted when used in the calculator. + +I found a few references on AliExpress (almost) matching all these contraints +and bought `this one `_. + +Which worked nicely: + +.. image:: {static}/images/numworks/test_ips_module.jpg + :alt: The IPS module works just fine, the image is correctly oriented. + :class: image-process-large-photo + +Obviously, the ribbon cable pinout is not the same as the original LCD module. +So an adapter had to be made. At first I thought I could just solder a bunch of +thin enameled wires, but this was not a very good idea; too hard to do +properly, without damaging the ribbon cable etc. So I decided to try the new +Flex PCB service JLCPCB (and others) now offer. Never did a Flex PCB design, +was a nice opportunity to give it a try. + +I did a simple design, with one side being connected to 30 pins FCP connector +on the Numworks side, and the other side with a 40pins FCP connecter soldered +onto the Flex PCB. + +The design, printed on paper to check dimensions, looks like this: + +.. image:: {static}/images/numworks/flex_design.jpg + :alt: Paper print of the flex-PCB adapter. + :class: image-process-large-photo + +The flex-PCB arrived a few days later: + +.. image:: {static}/images/numworks/flex_pcb.jpg + :alt: Flex-PCB adapter side-to-side with the paper print. + :class: image-process-large-photo + +Was not sure how it would support hand soldering the FCP connector, but it went +ok. + +.. image:: {static}/images/numworks/flex_pcb_connected.jpg + :alt: Flex-PCB adapter installed in the Numworks. + :class: image-process-large-photo + +It fits pretty nicely in the Numworks enclosure; the only modification I had to +make it to trim a bit some plastic pads to fit and center le LCD module. + +.. image:: {static}/images/numworks/repaired.jpg + :alt: The Numworks fully reassembled running the Omega firmware. + :class: image-process-large-photo + + +The Kicad design for the Flex-PCB is available `here `_.