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Good day,
Scenario:
LAUNCHXL2-TMS57012 (SPI Master)
Vectornav VN-200 (SPI Slave)
SPI Clock Speed: 8 MHz
SPI Mode 3
All pins are SPI functional (not GIO)
When I'm sending a request to the slave device, say: [ 0x01 0x05, 0x00, 0x00 ], the signals look good, the slave device understands and processes the request.
The expected answer to this request is: [ 0x00, 0x01, 0x05, 0x00, 0x00,0xC2, 0x01, 0x00 ]
The oscilloscope was always connected at the side of the slave device.
First two bytes of the request with SIMO (purple signal):
Byte: 0x01
Byte: 0x05
When the master device (LAUNCHXL2-TMS57012) is connected at the SOMI (green signal) pin we got the following byte in the answer (instead of 0x01), this obviously results in a 0x00 in the receive buffer:
... and the following byte in the answer (instead of 0x05), this obviously results in a 0x04 in the receive buffer:
Now comes the interesting part. When the SOMI pin is disconnected from the master device (LAUNCHXL2-TMS57012), the response bytes are not distorted and just like expected.
Expected 0x01 byte when SOMI (green signal) pin is disconnected from the master:
Expected 0x05 byte when SOMI (green signal) pin is disconnected from the master:
What has already been tried:
* Configure SOMI pin as open drain -> no effect
* Configure SOMI pin as pull up -> no effect
* Configure SOMI pin as pull down -> no effect
* Use of another SPI module (SPI1 and SPI3) -> always the same result
* Adding external pull up resistors to the SOMI pin -> no effect
Pull up resistors were selected according to: www.ti.com/.../slva689.pdf
* Using a different master device (LAUNCHXL2-TMS57012) of the same model -> always the same result
Good to know:
* The master and the slave device are powered by the same VCC and are using the same Ground
* The SPI cables which connect the two devices are foiled and the shield is also connected to Ground
* Cable length of the SPI cables is ~ 20 cm
Why does the master distort the SOMI signal like this?
Is this a known issue? What other SPI configurations may be selected to solve this?
Any help/advice would be much appreciated.
I look forward to reading from you.
Best regards
P.
