# Why are the resistors all different?

In my upcoming project, I am working with a Microcontroller called the Microchip PIC32MX695F512H and a Wifi Module called the Microchip MRF24WB0MA. Since I am new to electrical engineering, I've pretty much just been copying what I learned out of the data sheets on their websites and an already made schematic utilizing these two parts (the links are below). There are pictures I put as well that are down below.

Although all seems to be going well, I am having a few difficulties. On pin 7 of the Wifi Module (RESET), each schematic seems to be connected to different wires with different resistor types. One is connected to +3.3V with a 4.7k resistor, and the other is connected to ground with a 100k resistor. One other thing is that although on one of the schematics, pins 33, 34, and 35 all have a 4.7k resistor, only pin 33 has a 4.7k resistor on the other schematics.

I'm really sorry if these are newby questions, but I just recently got in the field of electrical engineering and need an answer DESPERATELY! Thank you!

• One other thing is that although on one of the schematics, pins 33, 34, and 35 all have a 4.7k resistor, only pin 33 has a 4.7k resistor on the other schematics. I think the first schematic is wrong. SPI is a push/pull, not an open drain bus. The datasheet specifically says pin 33 needs a pull-up, but not the others. – Passerby Mar 3 '15 at 4:50

4.7K is sort of a "strong" pull-up resistor, usually I see 10K to VCC (in your example, 3.3V)

The 100K is a weak "pull down" which avoids a floating pin during system reset or other odd occasions. This is common when you want deterministic start-up conditions on driven inputs/outputs - usually during power-up, or reset, as I mentioned.

Pull up resistors that I mentioned at the start, are used when you have an output which when "OFF" is just high impedance, or an open circuit, and when the output is exerted, it pulls "LOW" when it wants to be logic "ON". This allows you to interface with various voltage levels too, and is common for older circuitry - often referred to as Open Collector outputs.

In the case of inputs such as Reset, these are active LOW, which for noise-immunity reasons require a "HIGH" voltage to be "OFF", and when the pin is detected low (usually from a strong external current sink, or just power-off) then in this case the reset mode would be activated.

edit: Those schematics are pretty whack, and I can see why you are confused. In the second one, the reset is held low, but weakly, and the input signal is a "NOT reset-wifi", so if you do NOT want to reset the wifi, this signal has to be held high by something external. I guess the external signal may not be able to drive low, so the pull down resistor is there to ensure it goes low and holds the wifi in reset, until actively pulled high again.

edit2: The use of these resistors is basically related to what the device is interfacing with, the use-case scenarios (like what happens during a reset), and what the datasheet recommends. Except for what the datasheet recommends, the rest at optional/based on that application's requirements.

• historical note: 4.7K was a commonly specified resistor value when TTL logic was powered from 5V as it provides approximately 1mA current on a 5V drop. – Icy Oct 14 '15 at 12:11
• @Icy ah yes, that seems very sensible. For CMOS stuff nowadays we don't need very powerful signals at all, the total power usage is much better. 1mA continuous for each pin just to hold state with a resistor seems like a huge waste in comparison to modern logic circuits. – KyranF Oct 14 '15 at 23:00