SPI master RX problem with reception

Question

Hi, 
 I use SPI Master driver to communicate with a flash memory. 
 I use SDK 12 and i use nrf_drv_spi_transfer(...); to write and read over SPI. 
 My problem is the following : I write 4 bytes of command and 2 bytes of reception.
I received 6bytes with the two last bytes with my answer. 
 So my RX buffer have some useless data. 
 How can i only received usefull data ? 
 write on spi 
 
 TX buffer init 
 
 SPI configuration 
 
 SPI handler & buffer init 
 
 Result of the spi write/read 
 
 As you can see, the case 0 to 3 are useless data. I dont want it. 
 regards,

haakonsh · Accepted Answer

Hey Jean, 
 This is a feature of the SPI protocol. SPI is a simple generic full-duplex serial protocol. There are no preamble, headers or error correction mechanisms built in. There is only a Rx buffer and a Tx buffer which is accessed one SPI word at a time, usually one byte. 
 For every SPI clock cycle, one bit is transmitted from the Tx buffer, and one bit is received in the Rx buffer. The HW SPI peripheral or the SPI driver does not know that while you are transmitting, for instance, a control byte followed by a couple of address bytes, you are actually not interested in what you receive, they care only that you DO receive something, even if its 0x00. 
 You will have to implement a method for reading the Rx buffer yourself. You can either get the data you want by directly accessing m_rx_buf[4] or[5], or you can create an array pointer and point to the address of m_rx_buf[4]. Then your data would start in element 0 of the array pointer. 
 Cheers,
Håkon

SPI master RX problem with reception

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