SIGNAL-BUS-CLOCK TERMINATIONS In Embedded HARDWARE DESIGNS
There are two type of terminations
- Source Termination -Near Source
- End Termination -Near Receiver
Source termination works differently than end termination, but the net effect is the same. It works by controlling signal reflections, not preventing the reflections in the first place. Because of this, it only works if a driver output is feeding a single load.
The huge benefit of source termination is that it does not load down the driver like end termination does.
SERIES TERMINATION :
Most drivers have some resistance on it's output. That resistance is usually in the 20-30 ohm range. The sum of the output resistance of the driver and your resistor must equal your trace impedance in series termination.Source Termination may effect the rise time of the signal because of the RC time constant ie. Resistance of termination and capacitance of cable .
 |
| SERIES TERMINATION |
Reducing rise time will reduce the demand at the driver side which reducing the Instantaneous current demand on the driver [reducing ground bounce and EMI].A series terminated line does not stop reflections but reduces the amplitude of the ringing. If a source resistor is used with multiple loads, only the last device will see a cleaner edge.
OUTPUT RESISTANCE + TERMINATION RESISTANCE = TRACK IMPEDANCE
A reflection will occur when the termination resistor does not match the trace impedance
Parallel Trace Termination :
 |
| PARALLEL TERMINATION |
In parallel termination a resistor is connected to the ground or VCC parallel to I/O.Power dissipation is high during the operations at lower frequency ie. at lower switching rates .
TERMINATION RESISTANCE = TRACK IMPEDANCE
A reflection will occur when the termination resistor does not match the trace impedance .
INSTRUCTION WHILE PROBING THE SIGNAL
When checking signals on Scope you must probe at the receiver. Probing anywhere else will likely give you a distorted waveform and trick you into thinking that things are worse than they really are. Also, make sure that your ground clip is as short as possible.
Will include Differential pair terminations in next articles..
GooD LucK - AJ
0 comments:
Post a Comment