© 2000 Ubicom, Inc. All rights reserved. - 3 - www.ubicom.com SX IrDA Virtual Peripheral Implementation AN16 2.0 Programming Methodology The IrDA Virtual Peripheral consists of both interrupt and
mainline code. The interrupt service routine implements
the IrDA UART. Rather than implementing an application
by calling the IrDA sub-routines the code works on a call-
back system. Creating a new application involves imple-
menting certain functions which will be called by the IrDA
mainline  code.  This  will  become  clearer  in  the  section
describing the application layer.
An IrDA frame is processed one byte at a time through
every layer.
For reception the framing layer strips off the header and
passes the payload data one byte at a time to the pay-
load  layer  (part  of  the  LAP  layer).  The  payload  layer
strips  off  the  address  and  command  bytes  and  if  the
frame contains I data then it will pass the data one byte at
a time to the LAP layer which will pass it to the LMP layer.
The LMP layer will strip off the LMP address bytes and if
the frame contains IrComm data then it will pass it to the
application layer. At this point the frame has not yet been
validated by the frame-check-sequence (FCS) CRC nor
has it been validated as the correct frame number by the
LAP  layer.  Once  the  integrity  of  the  frame  has  been
checked  by  the  framing  layer  it  will  inform  the  payload
layer which will inform the LAP layer. The LAP layer will
verify the frame numbering and will inform the LMP layer
of  the  validity  of  the  frame  which  will  in-turn  inform  the
IrComm application layer.
The protocol stack is implemented as a number of layers,
each   one   based   on   a   state-machine.   All   the   state-
machines must operate in parallel and to achieve this the
state machines are event-driven. No state machine may
hold  up  the  processor  waiting  for  an  event,  but  instead
must complete its processing for the incoming event and
return.  The  event  handlers  for  a  layer  are  subroutines
where the name of the routine is predefined so that the
call to the event handler can be coded in the originating
code.
There are two types of events:
•    Asynchronous Events
•    Synchronous Events
Asynchronous events are events that are triggered by the
interrupt  service  routine  (ISR).  When  an  event  (e.g.  a
received IrDA byte) has been detected by the ISR will set
the  appropriate  flag  in  a  global  ISR  status  register.  The
main code is a loop that tests these flags and will call any
event handlers as appropriate.
Synchronous events are events that are generated by a
layer to inform a higher layer of a situation.
For example, when a byte has been received by the IrDA
UART in the ISR the IrDA UART data available flag will
be set. These asynchronous flags are polled by the main
routine and when the data available flag is detected the
data available event handler in the framing layer will be
called.  The  framing  layer  will  obtain  the  data  byte,  pro-
cess it, and return. However, if the framing layer detects
the byte as payload data then before returning it will call
the payload data available event handler in the payload layer.  The  payload  layer  will  process  the  data,  call  any
appropriate higher event handlers, and return. When the
end-of-frame byte is received by the framing layer it will
test  the  FCS  bytes  and  will  call  either  the  valid  or  the
error event handler in the payload layer. If appropriate the
payload layer will inform its higher layer and so on.
The following rules apply to inter-layer events or calls:
•    The routine name must start with the abbreviation of       the calling layer followed by a ‘2’ followed by the abbre-
      viation of the called layer.
•    The call instruction must be proceeded by a page in-       struction.
•    The return instruction from the called event must be a       ‘retp’ instruction.
•    The called code may change any global or shared reg-       isters.
•    Upon returning the bank is assumed to have been       changed.
•    A paged call may be replaced by a paged jump if direct- ly after the inter-layer call has been completed the code
will issue a return instruction to return from a inter-layer
call.