Does anyone know why these 4 resistors are there (attachment)? It seems like they are useless since connected in a loop to GND. Full schematics here: https://store.arduino.cc/arduino-due
1 Answer
Yes, they are redundant. They are elements of resistor arrays (multiple resistors in one package) that aren't used. They have just connected all unused pins of those packages to ground.
If you want to know why they would have unused elements, it's all to do with optimizing the manufacturing process. If you have, say some 100k arrays elsewhere in your design and you want to have maybe one or two other 100k resistors elsewhere, rather than adding another component of a discrete 100k resistor it's cheaper to use a 100k array and not use some of the elements. One of the factors in the cost of pick-and-place operations is the number of different components. By keeping the number of component types to a minimum they keep the manufacture costs to a minimum.
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great thanks, so if I understand correctly I could just use 1 resistor network (consisting of 4 resistors) to apply pullups to JTAG_RESET, JTAG_TDI, JTAG_TCK and JTAG_TMS and then get rid of the other 4 resistors? I'm designing a board based on this, hence the question.– danielCommented Dec 18, 2020 at 14:29
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Sure. A resistor array is just four individual resistors in a convenient package.– MajenkoCommented Dec 18, 2020 at 14:43
RN2Bwould be resistor network2, resistor elementB. There appear to be 4 resistors,AthroughD, inside one resistor-network package. You can seeRN5has the two unused resistors unconnected. The reason RN1 and RN2 have them connected to ground might have to do with noise. EDIT: looking at the eagle file, it seems like they connected them to ground to make the ground-plane flow better. My only question is why they didn't use a single resistor network here, instead of using only halve of two resistor networks.