Chaining 74HC595 and decoupling capacitors

[lavekravo]

Hi All,

I am planning to chain 8 74HC595 shift registers to control 32 NEMA17 stepper motors via A4988 drivers and an Arduino UNO. I was thinking of using the SparkFun shift register breakout available here: https://www.sparkfun.com/sparkfun-shift-register-breakout-74hc595.html.

However, I am a bit surprised that the breakout board does not contain a decoupling capacitor. So should I add/solder a 0.1uF decoupling capacitor on each 74HC595 breakout board between VCC (pin 16) and GND (pin 8)?

I am also planning to add a 100uF decoupling capacitor between VMOT and GND of each A4988 chips.

[van_der_decken]

The 0.1uF cap/IC won't do any harm and could do some good.

There's a very good discussion of the history and theory behind the choice of 0.1uF caps here:

Where did the value of 0.1uF for bypass capacitors come from?

[Grumpy_Mike]

I wrote a tutorial about decoupling, here on this forum, it shows why it is needed with any circuit:-

De-coupling Tutorial

De-coupling Once you get more than a handful of active components together you need to de-couple the power supply. What this is, and why, is described here. The problem When an otherwise good design works, but is erratic, unreliable or just goes crazy then nine time out of ten it is the power supply de-coupling. And you will find that the tenth time, turns out to be power supply de-coupling as well. So what is it, and why is it needed? Basically when an integrated circuit works it works by swi…

[lavekravo]

Thanks @van_der_decken and @Grumpy_Mike!
Glad to know I am on the right track. I will add the 0.1uF capacitors on 74HC595 breakout boards myself.

I am curious, why wouldn't these 74HC595 breakout boards already have a capacitor on them? For example, I am looking at the Adafruit A4988 breakout board and they already have a capacitor on them. So I was expecting the 74HC595 breakout boards to also have a capacitor, freeing me from the task of soldering one myself. Am I looking at the wrong board? Are there other 74HC595 breakout boards that already have a capacitor?

[LarryD]

• The 74HC595 is a CMOS chip that requires very little current.
  - It might be as simple as this low current requirement does not have the same issues that other families have when switching.
  - It It might be the designer overlooked this in their design.
  - It It might be the designer wanted to save a few cents per board

• In your designs, add one capacitor per power pin.

• The data sheet shows de-coupling.

• Avoid daisy chaining the power connections on your 74HC595 shift register boards.

• There are options out there:

[lavekravo]

@LarryD

• There are options out there:

Thank you @LarryD! This is a great option.

@LarryD

• Avoid daisy chaining the power connections on your 74HC595 shift register boards.

This is an interesting suggestion. What issues could arise from daisy chaining power connections through these boards? The two VCCs (and similarly the two GNDs) on the board should be just directly connected via the copper route on the board, right?

[LarryD]

• Power connections in a star configuration reduces noise on the rails.

[lavekravo]

I was expecting that to be the reason as well. Thanks for confirming @LarryD!

[lavekravo]

I created the following schematic diagram.

Questions:

1. Power supply for motors. The power source for motors is 24V 5A DC. Since each NEMA17 have a 1A current rating, if I add 3 more NEMA17, should I change the power supply to 24V 8A DC?
2. Power supply for chips. All 6 chips are getting their VCC/VDD from +5V of Arduino Uno. Is that okay, or should I consider adding another +5V power source for all the chips? Also, how many more chips can the Arduino Uno +5V pin supply?
3. Decoupling capacitors. Are the 6 decoupling capacitors correct? Should there be additional decoupling capacitors between pins 1 and 2 of each A4988?

[LarryD]

• Using an 8A power supply has no down side, suggest you use one.
• The stepper drivers need only 8mA on the logic power pin.
  If it were me, I would use an external 5v power supply to power the Arduino and external chips.
• All logic power pins need decoupling.

[lavekravo]

Thanks @LarryD.

@LarryD

• All logic power pins need decoupling.

Is this also for power pins connected to external power supply? For example, for the A4988 in my schematic diagram, the VDD and GND are connected to Arduino 5V and Arduino GND. Should there be a decoupling capacitors there too? Or should they be present only when I add the external 5V supply to power A4988?

[LarryD]

• Your module might include them, refer to the module's schematic.