SPI configuration

1. SPI Medium CPOL and CPHA Concept

         Often mentioned SPI There are four modes ：

         The above mode is only for SPI The clock of （SCK） To distinguish between ：

        CPOL That is, the polarity of the clock , Used to represent SPI When the bus is idle SCK Is it low level or high level , The low level is 0, The high level is 1;

        CPHA That is, the phase of the clock , Used to decide when to sample the signal , At the first jump edge or the second jump edge , Sampling at the first jump edge is 0, Sampling at the second jump edge is 1.

2. Multi slave configuration

         Multiple slaves can work with a single slave SPI Use with the host . The slave can be connected in normal mode , Or daisy chain connection .

2.1 routine SPI Pattern

         In normal mode , The master needs to provide a separate chip selection signal for each slave . Once the host is enabled ( Pull it down ) Piece of optional signal ,MOSI/MISO The clock and data on the line can be used for the selected slave . If multiple chip selection signals are enabled , be MISO Online data will be destroyed , Because the host cannot recognize which slave is transmitting data .

         As can be seen from the above figure , As the number of slaves increases , The number of chip selection lines from the host also increases . This will quickly increase the number of inputs and outputs that the host needs to provide , And limit the number of slaves that can be used . Other technologies can be used to increase the number of slaves in normal mode , For example, a multiplexer or a serial to parallel converter is used to generate a chip selection signal .

2.2 Daisy chain mode

         In daisy chain mode , The chip selection signals of all slaves are connected together , Data is propagated from one slave to the next . In this configuration , All slaves receive the same message at the same time SPI The clock . The data from the master is sent directly to the first slave , The slave provides data to the next slave , And so on .

         When this method is used , Because data is propagated from one slave to the next , Therefore, the number of clock cycles required to transmit data is proportional to the slave position in the daisy chain . For example, in figure 7 Shown 8 Bit system , In order to make the second 3 A slave can get data , need 24 Clock pulses , And conventional SPI In mode, just 8 Clock pulses .

 

3. The attachment

3.1 Advantages of daisy chain mode

         The biggest advantage of daisy chain is that it can connect multiple devices with limited signal transmission lines , Share the same service , And there are no problems such as bus competition and blocking , Because before a device transmits a signal to the next device , This signal can be modified .

3.2 Disadvantages of daisy chain mode

         The biggest drawback of daisy chain is that it is the serial transmission of signals , So once in the data link When some equipment breaks down , It is impossible for the lower priority devices to be served .