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AD7799 - problem with SPI verified with oscilloscope - help me understand?

Question asked by EGIF on May 18, 2017
Latest reply on May 23, 2017 by EGIF

Hello,

 

I am trying to establish communication between a Renesas RL78/G13 and an AD7799 - as detailed here:
AD7799 - Microcontroller No-OS Driver [Analog Devices Wiki] 

 

(My case is a little different in that I am using a proprietary RL78 PCB, but configured the same).

 

I cannot get communication to happen, it seems to be blocking at the transmission from the AD7799 to the RL78.

 

 

Here is a screenshot of that exchange

You can see in green, that is the response from the DOUT of the AD7799 to the SI of the RL78.

Decoding this manually shows the value expected (0x49) - but there is a slight problem with the last bit ...

 

Here I zoom on the bit in question at the end of the response from the AD7799:


 

Interpreting this - I look a the the rising clock edge and MSB - Notice the last bit - I marked it with "??" - this should be a 1 - but - the 1 does not last the entire clock cycle. This means I'm not really sure what it will be interpreted as - quite unexpected.

Since this is coming from the AD7799 - what can I do to correct this? 
Are there other issues at play I have not considered?

I must establish communication between the RL78/G13 and AD7799 to complete a prototype, any help with this issue is greatly appreciated!

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