badyjoke[m]: > <@badyjoke:matrix.org> Hi ! I'm having a weird issue with SPI. For context I use a ATSAM3X8 (Arduino Due) and I wrote my own HAL only for SPI. I'm reading data from a gyroscope but every byte I read is in double. I already played with baudrate/clock but it didn't change the behaviour. I also inspected with a probe and I have the expected result from the gyroscope and not any duplicated data. This is the implementation I have for send and receive data :
> 
> ```rust
>     /// Sends a single byte over the SPI bus.
>     ///
>     /// # Arguments
>     ///
>     /// * `word` - The byte to send.
>     ///
>     /// # Returns
>     ///
>     /// * `Ok(())` if the byte was successfully sent.
>     /// * `Err(ErrorKind::Overrun)` if an overrun error occurred.
>     /// * `Err(ErrorKind::ModeFault)` if a mode fault error occurred.
>     pub fn send(&self, word: u8) -> Result<(), ErrorKind> {
>         if self.spi.sr().read().ovres().bit_is_set() {
>             return Err(ErrorKind::Overrun);
>         } else if self.spi.sr().read().modf().bit_is_set() {
>             return Err(ErrorKind::ModeFault);
>         } else {
>             // For simplicity reasons we write only blocking version
>             while !self.is_tdr_empty() {}
>             unsafe {
>                 self.spi
>                     .tdr()
>                     .write_with_zero(|w| w.td().variant(word.into()))
>             }
>         }
>         Ok(())
>     }
> 
>     /// Receives a single byte from the SPI bus.
>     ///
>     /// # Returns
>     ///
>     /// * `Ok(byte)` if a byte was successfully received.
>     /// * `Err(ErrorKind::Overrun)` if an overrun error occurred.
>     /// * `Err(ErrorKind::ModeFault)` if a mode fault error occurred
>     pub fn receive(&self) -> Result<u8, ErrorKind> {
>         self.send(DUMMY_WRITE)?;
>         if self.spi.sr().read().ovres().bit_is_set() {
>             return Err(ErrorKind::Overrun);
>         } else if self.spi.sr().read().modf().bit_is_set() {
>             return Err(ErrorKind::ModeFault);
>         } else {
>             while !self.is_tx_empty() && self.is_rdr_full() {}
>             let rd = self.spi.rdr().read().rd().bits();
>             Ok(rd as u8)
>         }
>     }
> 
> ```
> 
> When reading the gyroscope ID and the gyroscope data and printing with defmt I get this :
> 
> ```console
> <lvl> Device: a9 - x a98c y 8cfd z fd07
> └─ spi::__cortex_m_rt_main @ src\bin\spi.rs:117
> <lvl> Device: 07 - x f0f0 y ebeb z 0606
> └─ spi::__cortex_m_rt_main @ src\bin\spi.rs:117
> <lvl> Device: a9 - x a91d y 1def z ef68
> └─ spi::__cortex_m_rt_main @ src\bin\spi.rs:117
> <lvl> Device: 68 - x e4e4 y c2c2 z 1a1a
> └─ spi::__cortex_m_rt_main @ src\bin\spi.rs:117
> <lvl> Device: a9 - x a958 y 5870 z 7064
> └─ spi::__cortex_m_rt_main @ src\bin\spi.rs:117
> ```
> 
> The expected device ID is A9. I'm still a beginner in embbeded and can't figure out this issue.
> Thank in advance !

Often with SPI, you're going to be transceiving data, so there's a latency of 1 byte after sending a command before the data comes in. What this means is that usually during the first byte transfer, the peripheral is also sending some data to you (it is usually either a DONT-CARE or a status byte). The actual response to i.e. a register read is usually sent in the second byte. In your driver, are you taking this into account?