Merging Arduino Pro Micro and USB Host Tiny 2.0

Im trying to merge an arduino pro micro and a USB Host Tiny 2.0 board (with a MAX3421E) together. And I came up with the following schematic:


(image seems to be too large to imbed)

The only differente to this is that I am using an AMS1117 instead of a MIC5219 on my breadboard.

There were a few questions I had.

  1. Is an electrolytic Capacitor (C14) really necessary for the 32U4?
  2. What is F1 and what is the purpose of this (if it limits the mA, is it really necessary?)
  3. The communication happens using SPI. Is it necessary to have a 5V to 3.3V converter (it works fine without on the bread board, but...?) If so suggestions on which?
  4. What is the purpose of the Diode (D2). Why is it not before the 5V connection as well?
  5. The USB to the 32U4 D+/D- uses 22ohm and the USB host uses 33ohm resistors on data. Why? is this just generic approximation? can they both be the same?

Below the original diagram for the arduino:

King_bob:
Im trying to merge an arduino pro micro and a USB Host Tiny 2.0 board (with a MAX3421E) together. And I came up with the following schematic:

http://www.kbob.org/Pictures/Artsy/Other/Other/USBuino.png
(image seems to be too large to imbed)

The only differente to this is that I am using an AMS1117 instead of a MIC5219 on my breadboard.

There were a few questions I had.

  1. Is an electrolytic Capacitor (C14) really necessary for the 32U4?
  2. What is F1 and what is the purpose of this (if it limits the mA, is it really necessary?)
  3. The communication happens using SPI. Is it necessary to have a 5V to 3.3V converter (it works fine without on the bread board, but...?) If so suggestions on which?
  4. What is the purpose of the Diode (D2). Why is it not before the 5V connection as well?
  5. The USB to the 32U4 D+/D- uses 22ohm and the USB host uses 33ohm resistors on data. Why? is this just generic approximation? can they both be the same?

Below the original diagram for the arduino:

http://www.kbob.org/Pictures/Artsy/Other/Other/ProMicro.png

  1. Unless you KNOW the 5V microUsb supply is clean, this is the only opportunity you have to reduce ripple on the 5V line.
  2. A fuse. It protects the USB power supply from overload if you try to draw too much current from the GPIO pins.
  3. You only need a converter if you plan to use 3.3V I2C devices.
  4. D2 is to block whatever you inject at the RAW pin from going back into whatever the USB is plugged into. RAW can be up to the spec of the regulator, so if you 7.2V at RAW, you don't want that to appear on the USB port.
  5. The series resistors give some impedance to the lines to reduce ringing of the high-frequency data lines. They can be the same. For no particular reason, I would use 33R.

I should point out the MAX3421E is a 3.3v chip


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SteveMann:

  1. Unless you KNOW the 5V microUsb supply is clean, this is the only opportunity you have to reduce ripple on the 5V line.
  2. A fuse. It protects the USB power supply from overload if you try to draw too much current from the GPIO pins.
  3. You only need a converter if you plan to use 3.3V I2C devices.
  4. D2 is to block whatever you inject at the RAW pin from going back into whatever the USB is plugged into. RAW can be up to the spec of the regulator, so if you 7.2V at RAW, you don't want that to appear on the USB port.
  5. The series resistors give some impedance to the lines to reduce ringing of the high-frequency data lines. They can be the same. For no particular reason, I would use 33R.

Firstly, thanks for your quick reply.

  1. I understand getting rid of the ripples, but I don't understand why they use one electrolytic capacitor and two ceramic ones, while the MAX3214E only uses two ceramic caps with the same capacitance. I usually see one electrolyte and one ceramic capacitor. Is there some sort of formula how it is decided what type of cap is used for this? What is this structure called to begin with? :slight_smile:
  2. Im trying to find an adäquate part number... would this be ok? MF-PSMF050X-2
  3. so I probably should use a converter since the MAX is 3.3V
  1. Can you just run the 32u4 at 8MHZ and 3.3V so you don't voltage convert between it and MAX?
  2. I would not mess with the serial resistors. Because the manufacturers say 22 or 33, I would just use those values. The serial resistance of D+ and D- are 45 ohm each I think.
    I would normally add a ferrite bead in line with the USB 5V to reduce EMI.

liuzengqiang:
3) Can you just run the 32u4 at 8MHZ and 3.3V so you don't voltage convert between it and MAX?
5) I would not mess with the serial resistors. Because the manufacturers say 22 or 33, I would just use those values. The serial resistance of D+ and D- are 45 ohm each I think.
I would normally add a ferrite bead in line with the USB 5V to reduce EMI.

  1. I was thinking about that... Would I need an 8Mz Quartz or could I keep the 16Mhz one? (It seems to me, that is should be doable without changing the quartz). What other aspects would that effect?
    ferrite bead? What How? I have never heard of ferrite on a PCB? Can you explain please?
  1. Yes you need an 8MHz crystal and caps. Using a 16MHz crystal at 3.3V would be overclocking the 32U4. Take a look at sparkfun's pro micro design. It has a 3.3V variant. The atmel doc also gives you reference design for when you use a 3.3V regulator to power your MCU.

Here is a digikey part number for the ferrite bead: MH2029-300YTR-ND.
I think the reason is that the bead smooths out current spikes that would couple to the data lines. I've always used one but now you ask, I should find where in USB specification it is mentioned.

I found the FT231X doc to mention it as a caution to reduce EMI from the IC to other devices. It may mean the high speed switching of TTL will cause the 5V line to change current at that speed thus causing emissions.

liuzengqiang:
3) Yes you need an 8MHz crystal and caps. Using a 16MHz crystal at 3.3V would be overclocking the 32U4. Take a look at sparkfun's pro micro design. It has a 3.3V variant. The atmel doc also gives you reference design for when you use a 3.3V regulator to power your MCU.

Here is a digikey part number for the ferrite bead: MH2029-300YTR-ND.
I think the reason is that the bead smooths out current spikes that would couple to the data lines. I've always used one but now you ask, I should find where in USB specification it is mentioned.

I found the FT231X doc to mention it as a caution to reduce EMI from the IC to other devices. It may mean the high speed switching of TTL will cause the 5V line to change current at that speed thus causing emissions.

I am guessing the ferrite is placed inline before or after the resistor on D+ and D- ?

King_bob:
I am guessing the ferrite is placed inline before or after the resistor on D+ and D- ?

I would put it as close to the connector as physically possible. You want to suppress any RF noise coming in.

The ferrite bead is on the VUSB as I stated earlier, not on the D+ or D-, right after the fuse.

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