Hello folks! I was browsing through the AVR hardware design considerations and I came across this
Shared Use of SPI Programming Lines
If additional devices are connected to the ISP lines, the programmer must be protected from any device,
other than the AVR device, that may try to drive the lines. This is important with the SPI bus, as it is
similar to the ISP interface. Applying series resistors on the SPI lines, as depicted in Connecting the SPI
Lines to the ISP Interface, is the easiest way to achieve this. Typically, the resistor value R can be 330Ω
Looking at the Eagle schematics for the Pro mini, I noticed that MISO/MOSI are not protected, but, there’s a 330Ω resistor on the SCK line attached to a Green LED.
My initial question is this: Is this resistor there for the purpose of protecting the SCK line, so that the AVR can be programmed even though you may have other devices attached on MOSI/MISO/SCK?
If that’s not the case, does this mean that we need to provide our own protection when programming the AVR via ISP?
The 330Ω resistor is not to protect the SCK line. This LED steps the voltage down a bit for the LED. If it’s suggested to use resistors on these lines during programming through this method then you may need to provide them, if the programmer device doesn’t have these protections in place already.
Hey Brandon, thanks for the reply! It’s not the programmer I’m thinking of, I’m going to be integrating https://www.sparkfun.com/products/13910 on a board with an atmega328p. Since the RFM uses MISO/MOSI/SCK, I have to have a way to program the atmega328p, without the RFM69HCW trying to drive the lines. I can always put something like a DIP switch and isolate the RFM69HCW but it’s not cost-effective and takes up a ton of space. Does this make sense?
I can see what you’re saying. That does sound like a cool project. Unfortunately, we don’t have any products similar to that which you could draw inspiration from. I would suggest that you could use some unbridged trace jumpers to isolate the RFM from those data lines. Then after programming, you can solder them to use the RFM module.