Reflection on SPI bus

spi termination

Hi all,

I need to use SPI bus on a big Printed Circuit Board (width: 70 cm). Near master microcontroller, there is a SPI DataFlash chip which must be accessed by full speed of AVR's SCK line (8 MHz).
Now I routing PCB and I am worry about refection on bus when master accessing DataFlash chip (about 50 cm float wire).
Is it reasonable to disconnecting float wire by transistor when accessing DataFlash or terminating line by a resistor?

Thank in advance,
Reza

spi bus termination

A suitable termination scheme depends on the topology. For a point-to-point connection in contrast to multi-drop, a source side series termination is sufficient (and also the optimal termination method).

spi series resistor

FvM said:

A suitable termination scheme depends on the topology. For a point-to-point connection in contrast to multi-drop, a source side series termination is sufficient (and also the optimal termination method).

Click to expand...

Thank you FvM,
I used point to point topology, how can i terminate the lines? is there any reference about this subject?

spi termination resistor

It's basic transmission line theory, but also discussed in many places in digital electronic literature. E. g. a lot of Altera application notes is dealing with the topic.

Basically, you need a source sided series resistor equal to the transmission line impedance. The pulse will be halved at the rising edge, but due to the +1 reflection factor at the open end, the voltage at the receiving end will have the original level. The back propagating wave will rise the voltage to full level up to the source.

When selecting the termination resistor, you should consider the driver output impedance, which usually has an amount up to several 10 ohms. The transmission line impedance can be expected in the 50 up to 100 ohm range, depending on the PCB technology.

Source side termination has the advantage of not causing statical power consumption or reduction of logical level in contrast to parallel termination at the receiver.