Atmel SAM4S-XPLD Atmel ATSAM4S-XPLD ATSAM4S-XPLD Hoja De Datos

 611

SAM4S [DATASHEET]

11100E–ATARM–24-Jul-13

A mode fault is detected when the SPI is programmed in Master Mode and a low level is driven by an external master on
the NPCS0/NSS signal. In this case, multi-master configuration, NPCS0, MOSI, MISO and SPCK pins must be
configured in open drain (through the PIO controller). When a mode fault is detected, the MODF bit in the SPI_SR is set
until the SPI_SR is read and the SPI is automatically disabled until re-enabled by writing the SPIEN bit in the SPI_CR
(Control Register) at 1.

By default, the Mode Fault detection circuitry is enabled. The user can disable Mode Fault detection by setting the
MODFDIS bit in the SPI Mode Register (SPI_MR). 

When operating in Slave Mode, the SPI processes data bits on the clock provided on the SPI clock pin (SPCK).

The SPI waits for NSS to go active before receiving the serial clock from an external master. When NSS falls, the clock is
validated on the serializer, which processes the number of bits defined by the BITS field of the Chip Select Register 0
(SPI_CSR0). These bits are processed following a phase and a polarity defined respectively by the NCPHA and CPOL
bits of the SPI_CSR0. Note that BITS, CPOL and NCPHA of the other Chip Select Registers have no effect when the SPI
is programmed in Slave Mode.

The bits are shifted out on the MISO line and sampled on the MOSI line. 

(For more information on BITS field, see also, the 

 below the register table; 

.)

When all the bits are processed, the received data is transferred in the Receive Data Register and the RDRF bit rises. If
the SPI_RDR (Receive Data Register) has not been read before new data is received, the Overrun Error bit (OVRES) in
SPI_SR is set. As long as this flag is set, data is loaded in SPI_RDR. The user has to read the status register to clear the
OVRES bit.

When a transfer starts, the data shifted out is the data present in the Shift Register. If no data has been written in the
Transmit Data Register (SPI_TDR), the last data received is transferred. If no data has been received since the last
reset, all bits are transmitted low, as the Shift Register resets at 0. 

When a first data is written in SPI_TDR, it is transferred immediately in the Shift Register and the TDRE bit rises. If new
data is written, it remains in SPI_TDR until a transfer occurs, i.e. NSS falls and there is a valid clock on the SPCK pin.
When the transfer occurs, the last data written in SPI_TDR is transferred in the Shift Register and the TDRE bit rises.
This enables frequent updates of critical variables with single transfers. 

Then, a new data is loaded in the Shift Register from the Transmit Data Register. In case no character is ready to be
transmitted, i.e. no character has been written in SPI_TDR since the last load from SPI_TDR to the Shift Register, the
Shift Register is not modified and the last received character is retransmitted. In this case the Underrun Error Status Flag
(UNDES) is set in the SPI_SR.