Philips LPC2194 User Manual page 180

Philips Semiconductors
ARM-based Microcontroller
When a device is a master, the start of a transfer is indicated by the master having a byte of data that is ready to be transmitted.
At this point, the master can activate the clock, and begin the transfer. The transfer ends when the last clock cycle of the transfer
is complete.
When a device is a slave, and CPHA is set to 0, the transfer starts when the SSEL signal goes active, and ends when SSEL goes
inactive. When a device is a slave, and CPHA is set to 1, the transfer starts on the first clock edge when the slave is selected,
and ends on the last clock edge where data is sampled.
SPI Peripheral Details
General Information
There are four registers that control the SPI peripheral. They are described in detail in "Register Description" section.
The SPI control register contains a number of programmable bits used to control the function of the SPI block. The settings for
this register must be set up prior to a given data transfer taking place.
The SPI status register contains read only bits that are used to monitor the status of the SPI interface, including normal functions,
and exception conditions. The primary purpose of this register is to detect completion of a data transfer. This is indicated by the
SPIF bit. The remaining bits in the register are exception condition indicators. These exceptions will be described later in this
section.
The SPI data register is used to provide the transmit and receive data bytes. An internal shift register in the SPI block logic is
used for the actual transmission and reception of the serial data. Data is written to the SPI data register for the transmit case.
There is no buffer between the data register and the internal shift register. A write to the data register goes directly into the internal
shift register. Therefore, data should only be written to this register when a transmit is not currently in progress. Read data is
buffered. When a transfer is complete, the receive data is transferred to a single byte data buffer, where it is later read. A read
of the SPI data register returns the value of the read data buffer.
The SPI clock counter register controls the clock rate when the SPI block is in master mode. This needs to be set prior to a
transfer taking place, when the SPI block is a master. This register has no function when the SPI block is a slave.
The I/Os for this implementation of SPI are standard CMOS I/Os. The open drain SPI option is not implemented in this design.
When a device is set up to be a slave, its I/Os are only active when it is selected by the SSEL signal being active.
Master Operation
The following sequence describes how one should process a data transfer with the SPI block when it is set up to be the master.
This process assumes that any prior data transfer has already completed.
1. Set the SPI clock counter register to the desired clock rate.
2. Set the SPI control register to the desired settings.
3. Write the data to transmitted to the SPI data register. This write starts the SPI data transfer.
4. Wait for the SPIF bit in the SPI status register to be set to 1. The SPIF bit will be set after the last cycle of the SPI data
transfer.
5. Read the SPI status register.
6. Read the received data from the SPI data register (optional).
7. Go to step 3 if more data is required to transmit.
Note that a read or write of the SPI data register is required in order to clear the SPIF status bit. Therefore, if the optional read
of the SPI data register does not take place, a write to this register is required in order to clear the SPIF status bit.
Slave Operation
SPI Interface
LPC2119/2129/2194/2292/2294
180
Preliminary User Manual
May 03, 2004