5 ms·

> A related trick is to put ferrite beads on MCU output lines; this takes the edge off fast-rising square wave signals, and can reduce needless inrush currents

> A related trick is to put ferrite beads on MCU output lines; this takes the edge off fast-rising square wave signals, and can reduce needless inrush currents when operating slower buses such as I²C or SPI. As /u/exmadscientist mentions, you probably shouldn't do this unless you know what you are doing. I also want to mention here that the fancier MCUs and SoCs will have configuration registers for drive strength, slew rate control and more for each output pin. If you're having a problem with ringing or whatever, you should definitely look to these settings, and see if you can fix this way. You might not need to make a PCB design change, which is always nice.

bsder 4y ago
If you genuinely need ferrite beads to solve your problems, the real solution is use Ethernet. Ethernet has already taken care of all the signal integrity issues as long as you follow the design guidelines from the application notes. You don't do this kind of thing with ferrite beads unless you need genuine high frequency isolation--generally because you are using a high precision ADC/DAC. A small resistor is more than good enough to round this off on an I/O pad. I always put a small (47 Ohm, 33 Ohm, or 22 Ohm) resistor in series with any digital "clock" line nowadays (SPI-SCLK, I2C--SCL, etc.). Modern chip processes can launch edges with GHz components onto clocks that are operating at KHz. I have had to debug quite a few "double clocking" faults and invariably just putting a small series resistor does a nice job of completely avoiding the problem.
- dfox 4y ago
  If you are driving something that really should not be a transmission line for your application from fast enough driver that makes it look like a transmission line then source termination (or just placing random resistor there as who the hell knows what the characteristic impedance of the thing really is) is the main thing that everybody should get from Handbook of Black Magic. Ferrite beads are for when you are driving external cables that should conform to some kind of interface standard and you have slew-limited driver.
  - wtfmcgrill 4y ago
    My immediate thought when I saw Handbook of Black Magic was "wonder what book has his nickname". "High Speed Digital Design: A Handbook of Black Magic" well I'll be damned, wish I had known about this a year ago.

bsder 4y ago

If you genuinely need ferrite beads to solve your problems, the real solution is use Ethernet. Ethernet has already taken care of all the signal integrity issues as long as you follow the design guidelines from the application notes. You don't do this kind of thing with ferrite beads unless you need genuine high frequency isolation--generally because you are using a high precision ADC/DAC. A small resistor is more than good enough to round this off on an I/O pad. I always put a small (47 Ohm, 33 Ohm, or 22 Ohm) resistor in series with any digital "clock" line nowadays (SPI-SCLK, I2C--SCL, etc.). Modern chip processes can launch edges with GHz components onto clocks that are operating at KHz. I have had to debug quite a few "double clocking" faults and invariably just putting a small series resistor does a nice job of completely avoiding the problem.

dfox 4y ago
If you are driving something that really should not be a transmission line for your application from fast enough driver that makes it look like a transmission line then source termination (or just placing random resistor there as who the hell knows what the characteristic impedance of the thing really is) is the main thing that everybody should get from Handbook of Black Magic. Ferrite beads are for when you are driving external cables that should conform to some kind of interface standard and you have slew-limited driver.
- wtfmcgrill 4y ago
  My immediate thought when I saw Handbook of Black Magic was "wonder what book has his nickname". "High Speed Digital Design: A Handbook of Black Magic" well I'll be damned, wish I had known about this a year ago.