Why the CSPI slave mode receive wrong data?

[wason]

Hi:

     I'm developing codes to communicating by using SPI module between two i.MX27 board .

One board works as the master/host CSPI and another works as the slave. I have writen master and slave code according the Freescale BSP's mxc_spi_test test code.    

    and my configuration is the following shape:

     MOSI is connected to MOSI

     MISO is connected to MISO

     SCK is connected to SCK

     SS is connected to SS.

     The slave  CSPI  when to receive data work as the interrupt mode.

      Firstly I use the loopback test to test the  CSPI. The master and slave CSPI work well, and they can write and receive same data.

      Secondly I connect two bosrd's CSPI interface. It work as :the master write some data and the slave receive its data from the master/host CSPI.

     Thirdly we can see receive data by HYPER TERMINAL.Sometimes the receive is OK.But sometimes the receive data is wrong.

     That is :I use SPI to write/send and receive data, the result is not correct. The 0x73 (01110011): the master/host send to the slave ,the slave received number is 0x37 (00110111) or 0x6E (01111110) and other data, the data is obviously 0x73 after shifting the data to the host from machine, the return value is the value after the shift, not the original value.

     I test another data. The master/host send to the 11114444,the slave received 1111CCCC ,CCCC or 3CCC and other data. The 1(0011 0001), 4(0011 0100) C(0100 0011),3(0011 0011). So the received data is the value after the shift.

     Every test I have run the SPI slave first and run the SPI master later. So the slaver receive first and master write data to the SPI later. The slave receive correct data after the master SPI write the data to the SPI. But sometime the slave SPI become receive wrong data and after few wrong data it becaome receive the correct data.  This wrong receive can appear often when I take the test.

      I donot known Why it is work incorrect? The master and slave clock polarity and phase are configured the same,the max_speed_hz = 1000000. Because I think the high speed would make CSPI work not well so I set the speed is 1000000. But I need the CSPI work as 4Mbps. The low speed are not working properly, not to mention the high speed.

     Follow is the HYPER TERMINAL print the message:

     The master write the data is 11114444, the slave should Data receive the 11114444.

Data received : 11114444

Data received : 11114444

Data received : 11114444

Data received : 11114444

Data received : 11114444

Data received : 11114444

Data received : 11114444

[fwriten data to SPI]ERROR: errno = 1, strerror = Operation not permitted

Data received : 1111CCCC

Data received : CCCC

Data received : 3CCC

Data received : CCCC

Data received : 3CCC

     The another test: the master send 1111444411114444,the slave should  receive 1111444411114444.

Data received : 1111444411114444

Data received : 1111444411114444

Data received : 1111444414444111

Data received : 111144441D4441

Data received : 1111444411114444

Data received : 1111444414444111

Data received : 11114444111144CC

Data received : CCCC3CCC

Data received : CCCCCCCC

Data received : CCCCCCCC

    Am I missing something here?

     Any pointers will be appreciated.

[wason]

Hi,

     From the data sheet of the i.mx27  I known that if the slave want read the data from the SPI master the slave should write same number of the data to the SPI and later read the data then.So according to the i.MX27 driver I write the same number dummy data to the SPI and then read same number of data from SPI master.  Is it all right?

     If anyone who can find some mistake please tell me without hesitate .

     Thanks you.

     Any pointers will be appreciated.

[Yuri]

Jianwang,

  Please look at the enclosed figure.

The SPI system can be considered of as a distributed 64-bit shift Register where half is in the master
and half is in the slave peripheral. When a transfer takes place this shift Register is circulated 32 bit positions,
so the data from the master ends up in the slave and the data from the slave ends up in the master ;
data is exchanged in full duplex fashion. Therefore, because of SPI nature (it may be considered as a ring, where
each clock provides both output bit and input one) the very first input bits of the RxFIFO may contain a garbage.

  Not all slave peripherals implement both data directions. In this case there is a data line from
the peripheral to the master but none from the master to the shift Register.  When the master SPI wants to read
this port, it initiates an SPI transfer by writing data to the SPI data Register. The slave cannot initiate transfer.

Also, please pay attention on the section 23.3.1 (Software Restrictions) of the i.MX27 Reference Manual :

• All reserved bits cannot be written and always read as 0.

• Writes to the TXDATA register are ignored when the CSPI module is disabled (SPIEN bit of CSPI

  ControlReg is cleared).

• The SPI Module Enable Control bit must be asserted before writing to other registers or initiating

  an exchange.

Have a great day,
Yuri

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