Hey! For dumping the SPI flash chip, the reset line needs to be low (since it’s active low). On this board, you added a switch to make it easy. How would you go about dropping that line low normally? I want to be able to do this on other boards in the future.
So, if you’re lucky, the board will already have a reset button or switch. If not, you have to add it. If i just needed to hit the reset pin once, i’d probably use a pair of tweezers or a small piece of wire to briefly short it to ground - But for a full flash dump, we need to keep it low for a while, so a better approach is necessary.
The next step past a pair of tweezers would be to solder a wire on to the board. You could leave the other end hanging and touch it to ground when you need to, or you could connect it to a jumper. Oftentimes when working on a board and knowing i’m going be tapping several signals, I’ll glue a strip of .1" headers at the edge of the board. anytime i need to access a signal, i’ll run a small wire from the pad on the board to one of the headers on the edge.
If you’re trying to not modify the board at all, something like a probe table or pcbite setup would let you contact the pins without having to solder.
For this class, i originally did the strip-of-headers-and-wires. You can see it in the pictures of the router on the website. When i switched to shipping kits to everyone for people to keep, i needed something quicker and more scalable - which is why i have that little custom pcb soldered in there on your boards. Also - spi flash dumping was the #1 spot for routers to get bricked, and i think it was often due to routers not being in reset while talking on the SPI lines. By using a slider switch, i made it something that’s pretty clearly either on or off and unlikely to get toggled mid-dump.
-joe
Thanks for the response! It makes sense why you did it but I just want to make sure I’ll be able to do this in the future. I tried doing this by hand (holding wires… I learned this is a bad idea) and thought I’d bricked it.
So, just to clarify for this target… there’s a resistor that goes to the reset line, which is the first easily exposed point for the reset line on the chip. My thought would be to use a pcbite to connect to this resistor and another one to ground. Then, you’d bridge these two points together. This would then set the reset line low successfully. Is this correct or am I misunderstanding something here?
First a bit of background. Lots of the components on a board are ‘pullup’ (to power) or ‘pulldown’ (to ground) resistors. Basically, if a wire or pin has a pullup resistor on it - and noone else is talking on it - it will default to a high voltage. Likewise, a wire with a pulldown will default to a low voltage when not otherwise being used.
On the TPLink, the reset signal needs to go high, but not immediately - it needs to delay a bit to let the rest of the chip power up. Some more complicated systems handle this with a separate reset controller, but ours just uses a pullup resistor and a capacitor. The pullup resistor pulls the reset pin high after power comes on - but the capacitor slows it down, so it takes a while for it to get to a high voltage.
So - to answer your question: The resistor is there to set the default value, which in this case is a pullup to a high voltage. To override that you need to short the pin in the other direction - to a ground pin. So yes - get a pcbite on the pin/resistor, and another on ground, and short them together. If/when you remove that short, the resistor will pull the line back up to high, and the system will boot.