The router is bricked, can the original NAND be replaced with spi flash?

1. Let's talk about the conclusion first

as long as the motherboard meets the following 3 conditions at the same time, the SPI-NAND Flash on the original machine can be replaced with SPI-NOR Flash to revive the "brick":

PCB reserve an 8-pin SPI-NOR pad (or you can fly wire to solve it).
SoC The boot pin can be switched to SPI mode (usually pulled up and down by 0 Ω resistor or empty pads).
you can get the "SPI version" of Breed/U-Boot and use the programmer to burn it into the new Flash first.

is indispensable, otherwise it will still not be lit.

2. Understand the principle in 30 seconds

When powering up

home routing SoC (MT7621/MT7628/AR934x, etc.), first decide where to find the 1st stage bootloader based on the level of the GPIO strap:

low-level → to go through the internal SPI-NOR controller
high-level → through internal SPI-NAND controller

original factory directly soldered the NAND particles and pulled the strap up in order to save costs.
what we need to do is pull down the strap, and then solder the NOR chip on it so that the CPU starts from NOR. Later, use Breed to brush the firmware that really runs the business.

3. Preparation before actual combat

4. Standard 6-step operation process

the following is an example of "Xiaomi Routing 3", other models change the soup without changing the dressing, and the strap designation is different.

dismantling the board to take pictures
take pictures of the front and back of the high-definition before dismantling the machine, so that it is convenient to fly the line and retrieve the resistor number in the future.
confirm that the SPI pad is usually left with an empty 8-pin pad on the back of the motherboard
silk-screen "U17", "J17", or "SPI_FLASH". Without an empty disk, you have to fly to the SPI0 pin (CLK/MOSI/MISO/CS#) of the SoC.
switching the boot mode
Xiaomi Routing 3 needs to reverse the two pairs of 0402 resistors, R139↔R137 and R124↔R126 (that is, change the strap foot that was originally pulled up to pull down).
operation: blow down the heat gun → the swap position → and solder it back.
Other models:
- Newifi D2: Changed R64 from 1→0 Ω to the opposite empty disk
- Pole Route 4: Remove R123 and weld it to the opposite empty disk
  In a word: Let the corresponding strap foot "ground".
offline programming Breed
clip the W25Q128 to the programmer, select "W25Q128JV", and flash the SPI version of Breed (with "spi" or "nor" in the file name) of the corresponding CPU.
burn it and write the ART partition (64 KB) to prevent 5G from being uncalibrated.
soldered back to the motherboard
NOR align 1 small dot with the silk screen triangle, and quickly complete it with hot air at 330 °C.
recommend removing the original NAND by the way to prevent bus conflicts.
power-on verification
plug in the TTL cable, baud rate 115200, you should be able to see the Breed start message on power-up:
```
U-Boot 1.1.3 (Sep 01 2022 - 23:18:42) Board: R3 SPI-NOR
```
hold down the Reset button and plug it in, the PC network port gets the 192.168.1.x address, and the browser enters 192.168.1.1 to flash the firmware.

5. Summary of common rollover points

phenomenon	Causes	solve
serial port has no output	1. strap did not succeed 2 NOR not welded/falsely soldered	whether the multimeter strap pin is <0.4 V; Heavy blowing NOR
output garbled characters	TTL Wire connection reverse or baud rate wrong	swap RX/TX to confirm 115200
Breed can be used, but the wireless signal is extremely poor after flashing the firmware	didn't write ART/EEPROM	"Update ART" item in Breed to revert to factory calibration
5G Disappear directly	firmware does not include a corresponding wireless driver	find the dedicated SPI version of Pandora/Padavan/OpenWrt

6. Is it worth changing?

only 16 MB of SPI-NOR space, it cannot fit the full version of OpenWrt, only the lite version or the old Maozi.
lose the official warranty after hard modification, the MAC address needs to be written back manually, otherwise the Mesh network and the official app will fail to recognize.
if the motherboard does not reserve an empty space and the strap foot is hidden in the BGA, it is extremely difficult to fly the line, so it is convenient to directly change the board or buy a new route.

one sentence :
"have an SPI disk + cut strap + can play a programmer", it is considered a practice, and the cost is 5 yuan;
"lack of disk + lack of schematic diagram + no heat gun", decisively give up, and the time cost is much higher than buying a new AX3000.