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content/numworks-lcd.rst
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| 1 ========================== | |
| 2 Numwork N0110 calculator | |
| 3 ========================== | |
| 4 | |
| 5 :Author: David Douard | |
| 6 :Category: Electronics | |
| 7 :Tags: Numworks, N0110, repair, calculator | |
| 8 | |
| 9 | |
| 10 My daughter's `Numwork <https://www.numworks.com>`_ N0110 graphing calculator | |
| 11 got fried when plugged in a cheap USB PSU. Black screen, heating up killing the | |
| 12 battery in a matter of a few tens of minutes. | |
| 13 | |
| 14 The design being originally Open Source (hardware and firmware), it makes sense | |
| 15 to try to fix it instead of just buying a new one (it is not a cheap | |
| 16 calculator). | |
| 17 | |
| 18 The schematic is pretty simple: built around a `STM32F730V8T6 <https://www.st.com/en/microcontrollers-microprocessors/stm32f730v8.html>`_, there are | |
| 19 only a few components surrounding the MCU: | |
| 20 | |
| 21 - an LCD (280x320, 2,8", ST7789V driver) driven using the 16 bits 8080 | |
| 22 interface, taking advantage of ST's `FSMC <https://www.st.com/resource/en/product_training/STM32F7_Memory_FSMC.pdf>`_ controller, | |
| 23 | |
| 24 - a 64MB flash (`AT25SF641 <https://www.renesas.com/us/en/products/memory-logic/non-volatile-memory/spi-nor-flash/at25sf641-64mbit-27v-minimum-spi-serial-flash-memory-dual-io-support>`_) to store apps and user data, | |
| 25 | |
| 26 - LiPo charger (`RT9526AGE <https://www.richtek.com/Products/Battery%20Management/Single%20Cell%20Li-Ion%20Battery%20Charger/RT9526A?sc_lang=en&specid=RT9526A>`_), | |
| 27 | |
| 28 - `RT9078 <https://www.richtek.com/Products/Linear%20Regulator/Single%20Output%20Linear%20Regulator/RT9078?sc_lang=en&specid=RT9078>`_ 2.8V voltage regulator to poser the MCU from the LiPo battery, | |
| 29 | |
| 30 - LCD backlight DC-DC converter (`RT9365GQW <https://www.richtek.com/Products/LED%20Driver/White%20LED%20Driver/RT9365?sc_lang=en>`_) | |
| 31 | |
| 32 - 3 LEDs plus a number of passives, keypad, reset button, | |
| 33 | |
| 34 - LiPo battery, | |
| 35 | |
| 36 - USB port surge protection diodes (`USBLC6-2SC6 <https://www.st.com/en/protections-and-emi-filters/usblc6-2.html>`_) | |
| 37 | |
| 38 | |
| 39 The list of fried devices was pretty much every active component: | |
| 40 | |
| 41 - MCU, | |
| 42 | |
| 43 - LCD, | |
| 44 | |
| 45 - 2.8v regulator, | |
| 46 | |
| 47 - USB protection diodes. | |
| 48 | |
| 49 | |
| 50 Due to the global chip shortage, I had to gamble on Aliexpress to find a | |
| 51 STM32F730. I bought one from `there <https://fr.aliexpress.com/item/1005004924466026.html>`_. Against all odds, the delivered chip | |
| 52 looks genuine and is working properly. | |
| 53 | |
| 54 So I replaced the MCU and the 2.8v regulator. At this point, I wasn't sure | |
| 55 about the status of the LCD. I did not know the USB protection diodes was dead, | |
| 56 so uploading a firmware was quite a challenge; USB based DFU was not working | |
| 57 (obvioulsy), but it was not obvious to understand why; I wasn't sure if a blank | |
| 58 STM32 should enumerate ok as USB device, or if bootstrping a firmware was | |
| 59 required before. | |
| 60 | |
| 61 I also did not want to upload Numworks' latest official firmware nor using | |
| 62 their WebDFU tools since they changes their licensing policy and moved to a | |
| 63 closed source model, official firmwares since version 16.3 now lock the | |
| 64 bootloader to prevent custom or open source firmware from being installed. | |
| 65 A complete explanation of this (in French) is `provided here <https://tiplanet.org/forum/viewtopic.php?f=97&t=25561>`_ | |
| 66 | |
| 67 So I started looking at the alternative firmware projects for the Numworks: | |
| 68 `Omega <https://getomega.dev/install/latest>`_, `Phi | |
| 69 <https://phi.getomega.dev/>`_, `Khi | |
| 70 <https://www-fourier.univ-grenoble-alpes.fr/~parisse/nws.html>`_ and `Upsilon | |
| 71 <https://lolocomotive.github.io/Upsilon-website/install>`_ (!) Plus starting | |
| 72 from a blank STM32 instead of an upgrade from an existing installation added a | |
| 73 bit of confusion. | |
| 74 | |
| 75 | |
| 76 After some time I got convinced I had successfully uploaded a firmware, but | |
| 77 noting showed up on the LCD: diagnostic of a dead LCD as well... | |
| 78 | |
| 79 The tricky part is that the LCD used in the calculator is pretty common on | |
| 80 principle: standard size, usual LCD controller. The very one used in the | |
| 81 calculator seems specificly made for the Numworks: specific ribbon cable and | |
| 82 specific pinout. Could not find a compatible device on usual sources. | |
| 83 | |
| 84 To validate I did indeed have a successfully replaced the MCU and uploaded a | |
| 85 working firmware, I did a nasty rastnest experiment with a ST7789V LCD module I | |
| 86 had around. | |
| 87 | |
| 88 The result was pretty ugly | |
| 89 | |
| 90 .. image:: {static}/images/numworks/rastnest.jpg | |
| 91 :alt: The test setup to connect a ST7789V based LCD module to the Numworks, | |
| 92 using a breadboard to interconnect the 2 interfaces. | |
| 93 :class: image-process-large-photo | |
| 94 | |
| 95 but I had *some* results: | |
| 96 | |
| 97 .. image:: {static}/images/numworks/partial.jpg | |
| 98 :alt: The LCD module shows half a screen with some content mixed with garbage. | |
| 99 :class: image-process-large-photo | |
| 100 | |
| 101 | |
| 102 First I was thinking the issue could be related to the speed of the | |
| 103 communications between the MCU and the LCD: the signal path was far from | |
| 104 optimal, with a ribbon cable, a breakout board, a bus of 10cm jump wires, a | |
| 105 breadboard, another bunch of 10cl jump wires, yet another bunch of jump wires, | |
| 106 another breakboard, and at last, the ribbon cable of the LCD module... So I | |
| 107 spent some time hacking the Omega source code to reduce the transmission speed; | |
| 108 but that did not help. | |
| 109 | |
| 110 To figure out what was wrong I ended up using my DSLogic logic analyzer. The | |
| 111 culprit was a pair of swapped jumpers (strangely there was only one pair of | |
| 112 messed connections). Since it was a pair of data lines above D7, it was | |
| 113 partially working. So the messed half display. | |
| 114 | |
| 115 With the 2 wires back in order, result was much better... | |
| 116 | |
| 117 .. image:: {static}/images/numworks/success.jpg | |
| 118 :alt: The LCD module shows the Numworks main screen... but flipped | |
| 119 :class: image-process-large-photo | |
| 120 | |
| 121 For some reason, this display showed the image reversed. Anyway, I was | |
| 122 confident enough to spend a bit more money buying a replacemement LCD module. | |
| 123 | |
| 124 Choosing a suitable replacement was not an easy task: | |
| 125 | |
| 126 - It needs to use the ST7789V controller. | |
| 127 | |
| 128 - It needs to be the proper size; seems easy, it's a standard 2.8" module... | |
| 129 well no, most of these modules are 66.95x47.80mm while the module in the | |
| 130 calculator is 66x50mm. Also the thickness must be about 2.1 or 2.2mm max. | |
| 131 | |
| 132 - It needs to have the proper ST7789V signals available, 16bits 8080 mode; | |
| 133 especially the Tearing Effect (TE) signal is used by the firmware to optimize | |
| 134 display refresh timing. | |
| 135 | |
| 136 - It needs to be an "12 o'clock" IPS LCD module (not a TN one) so the viewing | |
| 137 angle is adapted when used in the calculator. | |
| 138 | |
| 139 I found a few references on AliExpress (almost) matching all these contraints | |
| 140 and bought `this one <https://fr.aliexpress.com/item/1005004628969756.html>`_. | |
| 141 | |
| 142 Which worked nicely: | |
| 143 | |
| 144 .. image:: {static}/images/numworks/test_ips_module.jpg | |
| 145 :alt: The IPS module works just fine, the image is correctly oriented. | |
| 146 :class: image-process-large-photo | |
| 147 | |
| 148 Obviously, the ribbon cable pinout is not the same as the original LCD module. | |
| 149 So an adapter had to be made. At first I thought I could just solder a bunch of | |
| 150 thin enameled wires, but this was not a very good idea; too hard to do | |
| 151 properly, without damaging the ribbon cable etc. So I decided to try the new | |
| 152 Flex PCB service JLCPCB (and others) now offer. Never did a Flex PCB design, | |
| 153 was a nice opportunity to give it a try. | |
| 154 | |
| 155 I did a simple design, with one side being connected to 30 pins FCP connector | |
| 156 on the Numworks side, and the other side with a 40pins FCP connecter soldered | |
| 157 onto the Flex PCB. | |
| 158 | |
| 159 The design, printed on paper to check dimensions, looks like this: | |
| 160 | |
| 161 .. image:: {static}/images/numworks/flex_design.jpg | |
| 162 :alt: Paper print of the flex-PCB adapter. | |
| 163 :class: image-process-large-photo | |
| 164 | |
| 165 The flex-PCB arrived a few days later: | |
| 166 | |
| 167 .. image:: {static}/images/numworks/flex_pcb.jpg | |
| 168 :alt: Flex-PCB adapter side-to-side with the paper print. | |
| 169 :class: image-process-large-photo | |
| 170 | |
| 171 Was not sure how it would support hand soldering the FCP connector, but it went | |
| 172 ok. | |
| 173 | |
| 174 .. image:: {static}/images/numworks/flex_pcb_connected.jpg | |
| 175 :alt: Flex-PCB adapter installed in the Numworks. | |
| 176 :class: image-process-large-photo | |
| 177 | |
| 178 It fits pretty nicely in the Numworks enclosure; the only modification I had to | |
| 179 make it to trim a bit some plastic pads to fit and center le LCD module. | |
| 180 | |
| 181 .. image:: {static}/images/numworks/repaired.jpg | |
| 182 :alt: The Numworks fully reassembled running the Omega firmware. | |
| 183 :class: image-process-large-photo | |
| 184 | |
| 185 | |
| 186 The Kicad design for the Flex-PCB is available `here <https://git.sr.ht/~douardda/numworks-lcd-adapter>`_. |
