SPIS — Serial peripheral interface target with EasyDMA

SPIS supports SPI modes [0..3]. Modes CPOL and CPHA are set in the CONFIG register.

The SPIS peripheral shares registers and other resources with peripherals that have the same ID as SPIS. Before SPIS can be configured and used, all peripherals that have the same ID as SPIS must be disabled.

Disabling a peripheral with the same ID as SPIS will not reset any shared SPIS registers. Configure all SPIS registers to ensure they operate correctly.

See the Instantiation table in Instantiation for details on peripherals and their IDs.

SPIS implements EasyDMA for accessing RAM without CPU involvement.

For detailed information regarding the use of EasyDMA, see EasyDMA.

If RXD.MAXCNT is greater than TXD.MAXCNT, the remaining transmitted bytes will contain the value defined in the ORC register.

The END event indicates that EasyDMA is finished accessing the RAM buffer.

SPIS uses two memory pointers. RXD.PTR points to the RXD buffer (receive buffer) and TXD.PTR points to the TXD buffer (transmit buffer). Because these buffers are located in RAM, which can be accessed by both SPIS and the CPU, a hardware based semaphore mechanism is implemented to enable safe sharing.

The CPU must acquire the SPI semaphore before it can safely update the RXD.PTR and TXD.PTR pointers. The ACQUIRE task must be triggered for the CPU to receive the ACQUIRED event and have access to the semaphore. When the CPU has updated the RXD.PTR and TXD.PTR pointers, the CPU must release the semaphore before SPIS can acquire it.

The CPU releases the semaphore by triggering the RELEASE task, as illustrated in the following figure. Triggering the RELEASE task when the CPU does not have access to the semaphore will have no effect. See Semaphore operation for more information.

If the CPU is not able to reconfigure TXD.PTR and RXD.PTR between granted transactions, the same TX data will be clocked out and the RX buffers will be overwritten. To prevent this from happening, the END_ACQUIRE shortcut can be used. With this shortcut enabled, the semaphore will be handed over to the CPU automatically after the granted transaction has completed. This enables the CPU to update the TXPTR and RXPTR between every granted transaction.

The ENDRX event is generated when the RX buffer has been filled.

The RXD.MAXCNT register specifies the maximum number of bytes SPIS can receive in one granted transaction. If SPIS receives more than RXD.MAXCNT number of bytes, an OVERFLOW will be indicated in the STATUS register and the incoming bytes will be discarded.

The TXD.MAXCNT parameter specifies the maximum number of bytes SPIS can transmit in one granted transaction. If SPIS is forced to transmit more than TXD.MAXCNT number of bytes, an OVERREAD will be indicated in the STATUS register and the ORC character will be clocked out.

The RXD.AMOUNT and TXD.AMOUNT registers are updated when a granted transaction is complete. The TXD.AMOUNT register indicates how many bytes were read from the TX buffer in the last transaction. ORC (over-read) characters are not included in this number. Similarly, the RXD.AMOUNT register indicates how many bytes were written into the RX buffer in the last transaction.

The semaphore is a mechanism implemented inside the SPIS peripheral that prevents SPIS and CPU from accessing data buffers simultaneously.

By default, the semaphore is assigned to the CPU after the SPIS peripheral is enabled. An ACQUIRED event will not be generated for this initial semaphore handover. If the ACQUIRE task is triggered while the semaphore is assigned to the CPU, an ACQUIRED event will be generated immediately. The following figure illustrates the transitions between states in the semaphore based on the relevant tasks and events.

SPIS will try to acquire the semaphore when the STARTED event is detected. If SPIS does not obtain the semaphore, the transaction will be ignored and the semaphore is retained by the CPU. All incoming data on SDI will be discarded and the DEF (default) character will be clocked out on the SDO line throughout the transaction. This is also true if the semaphore is released by the CPU during the transaction. If a race condition occurs where the CPU and SPIS try to acquire the semaphore at the same time, as illustrated in lifeline item 2 in figure SPI transaction when shortcut between END and ACQUIRE is enabled, the CPU is given the semaphore.

If SPIS acquires the semaphore, the transaction will be granted. The incoming data on SDI will be stored in the RXD buffer and the data in the TXD buffer will be clocked out on SDO.

When a transaction is complete and CSN goes HIGH, SPIS will automatically release the semaphore and generate the END event.

SPIS can be granted multiple transactions in a row as long as the semaphore is available.

If the CPU tries to acquire the semaphore while it is assigned to SPIS, an immediate handover will not be granted. After the granted transaction is complete, SPIS releases the semaphore to the CPU. If the END_ACQUIRE shortcut is enabled and the CPU has triggered the ACQUIRE task during a granted transaction, only one ACQUIRE request will be served following the END event.

The CSN, SCK, SDI, and SDO signals associated with SPIS are mapped to physical pins according to the configuration specified in the PSEL.CSN, PSEL.SCK, PSEL.MOSI, and PSEL.MISO registers, respectively. If the CONNECT field is set to Disconnected, the associated SPIS signal will not be connected to any physical pins.

These registers and their configurations are only used when SPIS is enabled, and retained as long as the device is in System ON mode. See POWER — Power control for more information about power modes. When the peripheral is disabled, the pins behave as regular GPIOs, and use the configuration in their respective OUT bit field and PIN_CNF[n] register. Only configure PSEL.CSN, PSEL.SCK, PSEL.MOSI, and PSEL.MISO when SPIS is disabled.

Before enabling SPIS, the pins used by SPIS must be configured in the GPIO peripheral as described in GPIO configuration before enabling peripheral. This ensures that the pins are driven correctly if SPIS becomes temporarily disabled, or if the device enters System OFF mode. This configuration must be retained in the GPIO for the selected pins to be recognized by an external SPI controller.

The SDO line is set as an input, with input buffer connected, as long as SPIS is not selected with CSN. This can cause additional current consumption if the SDO line is left floating. To avoid this, it is recommended to connect a pull-up or pull-down resistor on the SDO line when it is not driven by an external SPI controller.

Only one peripheral can be assigned to drive a particular GPIO pin at a time. Failing to do so may result in unpredictable behavior.