Mercurial > whatever / file revision

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 Numworks N0110 calculator full repair
=======================================

:Author: David Douard
:Category: Electronics
:Tags: Numworks, N0110, repair, calculator
:slug: numworks-lcd


My daughter's `Numworks <https://www.numworks.com>`_ 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 <https://www.st.com/en/microcontrollers-microprocessors/stm32f730v8.html>`_, 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 <https://www.st.com/resource/en/product_training/STM32F7_Memory_FSMC.pdf>`_ controller,

- 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,

- LiPo charger (`RT9526AGE <https://www.richtek.com/Products/Battery%20Management/Single%20Cell%20Li-Ion%20Battery%20Charger/RT9526A?sc_lang=en&specid=RT9526A>`_),

- `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,

- LCD backlight DC-DC converter (`RT9365GQW <https://www.richtek.com/Products/LED%20Driver/White%20LED%20Driver/RT9365?sc_lang=en>`_)

- 3 LEDs plus a number of passives, keypad, reset button,

- LiPo battery,

- USB port surge protection diodes (`USBLC6-2SC6 <https://www.st.com/en/protections-and-emi-filters/usblc6-2.html>`_)


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 <https://fr.aliexpress.com/item/1005004924466026.html>`_. 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 <https://tiplanet.org/forum/viewtopic.php?f=97&t=25561>`_

So I started looking at the alternative firmware projects for the Numworks:
`Omega <https://getomega.dev/install/latest>`_, `Phi
<https://phi.getomega.dev/>`_, `Khi
<https://www-fourier.univ-grenoble-alpes.fr/~parisse/nws.html>`_ and `Upsilon
<https://lolocomotive.github.io/Upsilon-website/install>`_ (!) 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 <https://fr.aliexpress.com/item/1005004628969756.html>`_.

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 <https://git.sr.ht/~douardda/numworks-lcd-adapter>`_.