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Output Issue with LMX2594 Serial Data

cooperhow

New Member
In our RF LNB, three LMX2594 RF synthesizers are connected to the Microcontroller SPI Bus. All three LMX2594 chips share the SPI CLK, MISO and MOSI lines (called SCK, MUXout and SDI in the datasheet). Each LMX2594 device has its Chip Select signal from the microcontroller (CSB). We can read and write if there is only one device populated. If we populate all three devices, we can’t read the registers from any of them. We know programming each by looking at the RFoutput is doable. When probing with the oscilloscope, there are good-quality SPI CLK and SPI MOSI signals but no MISO signal. What is the reason of corrupted MISO signal?
 

Pommie

Well-Known Member
Most Helpful Member
My guess would be bus contention. As all are outputs they must all be acting at once regardless of the CS line. Try using a diode on each MISO with Anode to chip and Cathodes to a single pullup resistor. This is assuming that they output a logic high when idle - if logic low then swap diodes around and use a pull down. For more info google diode logic gates.

Mike.
Note, the above is only a guess. Some of the RF guys here may have better knowledge of this.
Edit, another solution would be an AND gate for each chip. inputs MISO and CS for each chip and all outputs joined. This assumes that only 1 CS line can be high at once.
 

Pommie

Well-Known Member
Most Helpful Member
Would slowing the transfer rate and using a diode OR gate be a possible solution. Wouldn't any solution with active outputs cause the same contention?

Mike.
Edit, Ahha, one 3 input or gate for three chips.
 

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