Lpc2000

Typically in microcontroller SSEL line is used to select master or slave mode for microcontroller SPI port.
Generally is is pulled high or low according to requirement. in some cases you can use this pin as GPIO  (Not in all cases for example :
look at following part from philps LPC2292/2294 document ...
SPI Interface 182 May 03, 2004

Philips Semiconductors Preliminary User Manual

LPC2119/2129/2194/2292/2294 ARM-based Microcontroller

PIN DESCRIPTION

Table 114: SPI Pin Description

Pin Name Type Pin Description

SCK1, SCK0

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.

SSEL1, SSEL0 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.

Note: LPC2119/2129/2194/2292/2294 configured to operate as SPI master MUST select

SSEL functionality on an apropriate pin and have HIGH level on this pin in order to act

as a master.

MISO1, MISO0

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.

MOSI1, MOSI0

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.



K B Shah
  ----- Original Message ----- 
  From: Sean 
  To: lpc2000@yahoogroups.com 
  Sent: Tuesday, March 21, 2006 5:29 AM
  Subject: Re: [lpc2000] Interfacing two peripherals on the same SPI port... SSEL function?



  The SSEL line is the dedicated CS signal per device.  The output SSEL line 
  on the uC is just an example, you can use any GPIO for this functionality 
  to control multiple devices, or if you want several then you can use a 
  decoder chip i.e. 2-to-4 or 3-to-8 to conserve on GPIO lines.

  -- Sean

  At 05:21 3/21/2006, you wrote:
  >Dear,
  >
  >How should I use the SPI port on an LPC2200 processor if I want to
  >interface two or more peripherals on it?
  >
  >I thought that every slave peripheral should have a dedicated Chip
  >Select signal, and the one for which CS is low would be allowed to
  >talk/hear.
  >
  >BUT, there is only one SSEL signal per SPI bus on the LPC. Should I
  >use GPIO pins to create a CS for each peripheral?
  >
  >Many thanks in advance.



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