NXP Semiconductors LPC24XX UM10237 - Manual (Page 530)

UM10237_4 © NXP B.V. 2009. All rights reserved.
User manual Rev. 04 — 26 August 2009 530 of 792
NXP Semiconductors
UM10237
Chapter 19: LPC24XX SPI
If the SSEL signal goes active, when the SPI block is a master, this indicates another
master has selected the device to be a slave. This condition is known as a mode fault.
When a mode fault is detected, the mode fault (MODF) bit in the status register will be
activated, the SPI signal drivers will be de-activated, and the SPI mode will be changed to
be a slave.
If the Px.y/SSEL/... pin is assigned the SSEL function in Pin Function Select Register 0,
the SSEL signal must always be inactive when the SPI controller is a master.
Slave Abort
A slave transfer is considered to be aborted, if the SSEL signal goes inactive before the
transfer is complete. In the event of a slave abort, the transmit and receive data for the
transfer that was in progress are lost, and the slave abort (ABRT) bit in the status register
will be activated.
6. Pin description
Table 460. SPI pin description
Pin
Name Type Pin Description
SCK Input/
Output Serial Clock. The SPI is a clock signal used to synchronize the transfer of data
across the SPI interface. The SPI is always driven by the master and received
by the slave. The clock is programmable to be active high or active low. The SPI
is only active during a data transfer. Any other time, it is either in its inactive
state, or tri-stated.
SSEL Input Slave Select. The SPI slave select signal is an active low signal that indicates
which slave is currently selected to participate in a data transfer. Each slave has
its own unique slave select signal input. The SSEL must be low before data
transactions begin and normally stays low for the duration of the transaction. If
the SSEL signal goes high any time during a data transfer, the transfer is
considered to be aborted. In this event, the slave returns to idle, and any data
that was received is thrown away. There are no other indications of this
exception. This signal is not directly driven by the master. It could be driven by a
simple general purpose I/O under software control.
On the LPC2400 (unlike earlier NXP ARM devices) the SSEL pin can be used
for a different function when the SPI interface is only used in Master mode. For
example, pin hosting the SSEL function can be configured as an output digital
GPIO pin and used to select one of the SPI slaves.
MISO Input/
Output Master In Slave Out. The MISO signal is a unidirectional signal used to transfer
serial data from the slave to the master. When a device is a slave, serial data is
output on this signal. When a device is a master, serial data is input on this
signal. When a slave device is not selected, the slave drives the signal high
impedance.
MOSI Input/
Output Master Out Slave In. The MOSI signal is a unidirectional signal used to transfer
serial data from the master to the slave. When a device is a master, serial data
is output on this signal. When a device is a slave, serial data is input on this
signal.