Re: SPI bus problems with PIC microcontroller



I am using PIC18F6410 and interfacing it to Linear technology's
LTC2704 ( 16 bit DAC ).

I am checking the data signals on the SDO pin ( MOSI pin of the SPI
bus ) using a USB based Logic Analyzer.

When the pins of the Logic Analyzer are attached to the SDO pin, the
DAC is able to recognise the SPI command and acts accordingly, but
when I remove the logic analyzer connections, the DAC doesn't
recognise the SPI commands and acts like it is dead.

The voltage ouput on the pins of this logic analyser is about 1.157V
( between logic analyser pin and its GND pin).

Is there any fuse setting I am missing ? Is there any clock speeds
that I need to look into. I am running the PIC microcontroller at
20Mhz at present. Please help.

Maybe you have a voltage mis-match at the interface. Check the data sheets
for voltage requirements.

Maybe you have a line termination problem. Try using a pull up or pull down
resistor. Try using a cap to simulate the logic probe. Then try to
understand the differences.

Maybe your DAC can't take clocks at the rate you're driving it. Check the
data sheets for clock requirements.

Try using a multichannel scope to look at the signals. A logic analyzer
doesn't show you the situation in the analog domain. You might see
overshoot, ringing, or slow edges that you can't see with a logic analyzer.

JJS


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