36 votes

SPI bus termination and pull-up resistors

At 1 MHz and 50 mm (2 inches) you don't need terminations. You have ordinary on-board digital signals, and not even very fast ones. You don't need pullups or pulldowns on SPI lines. When used, SPI ...
Olin Lathrop's user avatar
16 votes
Accepted

Are special terminals required for connecting thermocouple wires to a PCB?

Using a terminal block made with ordinary materials is quite sufficient for a relatively modest accuracy system as you're aiming for. The cold junction compensation depends on the cold junction ...
Spehro Pefhany's user avatar
16 votes
Accepted

Can we reduce power consumption of digital interfaces by using high impedance transmission lines?

While, in theory, high impedances would reduce power dissipation for the same voltage swing, there are several important issues in practice. 1) It's the power, not the voltage, of a signal that ...
Neil_UK's user avatar
  • 167k
15 votes

Impedance matching and termination resistor

Imagine an infinite board with an infinite trace on it. Apply 1V to the trace. What happens is that an impulsive wave, a step, propagates down the trace. If the current needed to keep this wave going ...
John Doty's user avatar
  • 4,389
14 votes
Accepted

Why is the signal amplitude low on the oscilloscope even though the impedance is matched?

Do not confuse power transfer with voltage transfer. If the source is indeed 50 ohms, and your scope is set to 50 ohms, it becomes a voltage divider and what you see is half the voltage that you will ...
Trevor_G's user avatar
  • 46.7k
12 votes
Accepted

Are termination resistors needed for UART, I2C and SPI?

Take 50 MHz clock SPI. 50 MHz has a fundamental wavelength of 6 metres but you could argue that due to the fast rising edges everything up to the 5 th harmonic is potentially capable of disrupting ...
Andy aka's user avatar
  • 458k
12 votes

Does it matter which 2 nodes on a CAN bus you terminate?

Each beginning has its end. This can't be answered, because the question is not correct. What you have is the transmission line - a twisted pair that has two ends, and that's the place where the ...
Marko Buršič's user avatar
11 votes

Driver impedance of a microcontroller pin, for series termination?

It's not the frequency of the signal that's sent down a PCB trace, but the risetime that's important. A 21cm PCB trace is about 30cm 'air equivalent', and as the speed of light is 1nS/ft (in units ...
Neil_UK's user avatar
  • 167k
11 votes

Does it matter which 2 nodes on a CAN bus you terminate?

It doesn't matter which nodes are at the ends, but it does matter that the terminators are at each end. The bus is a transmission line. To keep edges from reflecting at the ends of the cable, the ...
Olin Lathrop's user avatar
11 votes
Accepted

The CAN bus in this vehicle has only one 120 ohm termination resistance. What problems would this provoke?

From a professional point of view, this is a critical problem. Missing termination will cause energy bouncing back at the end which isn't terminated. This could lead to strange random noise on the ...
Lundin's user avatar
  • 20.3k
11 votes
Accepted

When do you need a complex termination?

So the characteristic impedance is a complex number. But why is never a complex termination used? When \$j\omega\$ is significant, the formula reduces to this: - $$Z_0 = \sqrt{\dfrac{j\omega L}{j\...
Andy aka's user avatar
  • 458k
10 votes

Why is Bob Smith termination for Ethernet recommended if it's wrong?

I "Bob Smith" with 75 ohms and I believe you should too. You can find a few corner cases where this termination doesn't work as well as something else, but overall it is the most robust if you do ...
Jerry's user avatar
  • 101
10 votes
Accepted

SPI bus connections - to loop or not to loop?

Loop back? NO. If the line is long enough to need termination (longer than risetime*c/10 or so), then drive it strongly enough to end terminate it properly, and match the line and termination ...
Neil_UK's user avatar
  • 167k
10 votes
Accepted

How Does an Adjustable Transistor Size CMOS Driver Work?

The approach to do this is alluded to in the datasheet snippet you linked, which mentions "fingers", or separate parallel gates in a wide MOSFET that forms the output driver. These fingers ...
nanofarad's user avatar
  • 20k
10 votes
Accepted

Should long PCB traces be treated as transmission lines?

Should I treat this PCB trace as a transmission line If your signal goes up to gigahertz range, then yes, your 5-7cm trace must be treated as a transmission line and meet your characteristic ...
Ale..chenski's user avatar
  • 40.7k
9 votes

USB trace impedance calculations, with termination resistors

question 1: This doesn't seem right The default Er is 4, but usually the ER for standard FR4 is around 4.6. Although for this calculation you will need the effective Er because the top of the trace ...
555's user avatar
  • 369
9 votes

Should long PCB traces be treated as transmission lines?

Whether a PCB trace is treated as a transmission line or not is purely based on the propagation delay of the trace and the rise time of the signal. Please see this nice article from Altium. A summary ...
Confused's user avatar
  • 2,263
8 votes
Accepted

Will this selectable CAN BUS termination circuit work?

It looks like it should work if you're not pushing the CAN bus to the limit. You can definitely turn on the LED with 3.3 V since the maximum LED voltage is 1.5 V. It needs about 5 mA, so you can ...
Olin Lathrop's user avatar
8 votes

Driver impedance of a microcontroller pin, for series termination?

Conventional wisdom on transmission lines tells us that you need to match driver to line and line to termination. However, this conventional wisdom creates a problem with logic circuits because a 50 ...
Andy aka's user avatar
  • 458k
8 votes
Accepted

How and why is impedence matching done for a CAN bus?

The "terminators" of a CAN bus serve two purposes: They terminate the transmission line. The type of CAN you are referring to is intended to be implemented as a twisted pair. 120 Ω is roughly ...
Olin Lathrop's user avatar
8 votes

Can we reduce power consumption of digital interfaces by using high impedance transmission lines?

This termination resistance closes the current loop between two differential signal lines and creates a decent amount of power loss so that low voltage amplitudes are required to reduce power ...
pjc50's user avatar
  • 46.7k
8 votes

Impedance matching and termination resistor

I know this resistor is for absorbing the wave energy, but how we can relate this resistor to the line impedance matching? The characteristic impedance of the line is the ratio of voltage to current ...
The Photon's user avatar
  • 130k
7 votes
Accepted

RS422, RS485 Termination Resistors

When you send a high speed signal down a cable, the current that initially flows is dictated by the voltage applied AND the characteristic impedance of the cable. For the types of cable recommended ...
Andy aka's user avatar
  • 458k
6 votes

Are special terminals required for connecting thermocouple wires to a PCB?

Any change in material will introduce an error on the signal. However if the changes are the same (in terms of both material and temperature) on both terminals then those errors will cancel out. So ...
Andrew's user avatar
  • 6,902
6 votes
Accepted

How many (m)W should 120 ohm CAN bus terminators have?

You seem to have your answer from the comments, so this is mainly for posterity purposes. Your calculations are correct, at a max of 5V differential voltage you would have ~42mA which would dissipate ...
Redja's user avatar
  • 660
6 votes

Calculating series resistors for I2C/SPI lines to reduce EMI and ringing

So many questions. Yes, when the signal is not bidirectional, place the resistor close to the source. You should also place a capacitor close to the load. You can start with the capacitor not loaded ...
user57037's user avatar
  • 29.3k
6 votes
Accepted

Check of my first Eagle boards

As mentioned in comments, some stitching vias between the two ground planes would help eliminate ground islands. I'd also eliminate these ground fingers that could act as little antennas: Watch out ...
The Photon's user avatar
  • 130k
6 votes

Serial resistance for SPI bus (33 ohms, 50 ohms....)

The general idea is that the bus has a particular characteristic impedance, typically in the range of 50-70Ω, but the driver output impedance is usually much lower, more like 15-30Ω. The ...
Dave Tweed's user avatar
  • 173k
6 votes

Serial resistance for SPI bus (33 ohms, 50 ohms....)

Proposals Proposals like, put 100nF on the VCC, or put 33 Ohms on the communication line are mostly not based on a basic understanding of what's really happening. That's mainly the reason why the "...
Finkman's user avatar
  • 114

Only top scored, non community-wiki answers of a minimum length are eligible