"I knocked off a resistor, but I dont know why it doesnt work"
See a problem here
You need to replace that resistor. You can either get an 0603 (I think) SMD version of it, and solder it back into place, or use a normal, 1/8w, 10k ohm resistor.
Here's why you need that resistor (thanks nB):
QUOTE
The LPC (Low Pin Count) interface is specifically designed for peripheral expansion (much like the PCI bus, of which it is a subset). Now, many of you are likely used to the digital electronics concept of a TTL driver, the chip drives a voltage out to represent a "1" and pulls the signal to ground to represent a "0". While this method works fine when you have one chip talking to another, or even when you have one chip talking to many, it doesn't work so well when more than one chip may try talking at the same time.
What happens when you have one chip try to drive a 0 and another trying to drive a 1 at the same time? A lot of current flows across what is essentially a short circuit and one of the two chips dies (if you're lucky). How then do you put multiple devices on a bus and guarantee that you will never have this happen? Add a bunch of control circuitry and address lines. But what if you don't have the pins available to add the control lines? What if your little PIC micro can't drive the complex circuitry required? There is a simple solution: take away the ability of the chips to drive a "1".
So now things get interesting. When you make all your devices "open collector" you have essentially removed the transistor that puts out a voltage to drive a 1, but left in place the transistor that pulls the line to ground in order to output a 0. In this situation if more than one device wants to talk the data may get corrupted, but no damage will occur and you can simply ask the device to re-transmit. At this point you may be asking: yeah, but how do I transmit if I cant drive a 1? Those resistors are the answer.
Each of the LAD lines (as is the SDA line of the I2C bus) are capable of being connected to many devices, all of which have an open collector. The little resistors are 10K ohm pull-up resistors, that is to say they pull the lines voltage up when no chip on the bus is trying to pull it low. While this approach limits your total signal speed (33MHz on the LPC), it vastly reduces interface overhead and makes simple data busses easy to implement on small controllers.
The I2C bus is another example: The PIC, EEPROM, Video Encoder, and some of the LCDs all connect to the I2C bus, again through the same open collector principal.
The reason you can not simply replace a damaged resistor with a blob of solder (as I have seen some suggest), is that were you to do this, your pull-up would effectively be trying to drive as much current as it could into any chip trying to drive the bus to a 0, and in most conditions result in damage to any chips connected (mod, MCPx).
Update: Apologies for a factual error in the piece above. The LPC interface is not an open collector interface, it is a bi-directional interface requiring a turnaround where neither chip drives a signal, yet the bus must remain high. The specifics about open collector still applies, however. Thanks,
-nB
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Topic: Modchip Install Problem (Read 102 times)