© 2000 Ubicom, Inc. All rights reserved. - 8 - www.ubicom.com AN16 SX IrDA Virtual Peripheral Implementation 3.3 LAP LAYER The primary purpose of the link-access-protocol (LAP) layer
is  to  manage  the  link  to  provide  a  connection-based  bi-
directional reliable data transfer service. It manages the dis-
covery process, the connection process, and the transfer of
data.
The LAP layer implementation is secondary only with two
additional primary features:
•    Support for initiating XID discovery.
•    Support for sending unnumbered-information frames.
The LAP layer is implemented in the IrDA Virtual Peripheral
as two parts – payload layer and LAP layer. 3.3.1  Discovery In  order  for  an  IrDA  device  to  open  a  connection  with
another IrDA device it must first know it is there and the dis-
covery  process  must  be  able  to  cope  with  multiple  IrDA
devices in range. The discovery (XID) frames pass a 32-bit
device  address,  hint  bits  as  to  the  type  of  device,  and  a
‘nickname’ string that can be displayed to the user.
Both  command  and  response  XID  processes  have  been
implemented so that the SX can discover other devices and
be discovered itself.
The discovery process is outside of any connection and so
all  frames  relating  to  the  discovery  process  are  sent  at
9600bps with a minimum turnaround of 10ms.
When a device wishes to discover devices in range it will
transmit a XID command frame indicating slot 0 of N slots
(typically 8). Every device receiving a XID command frame
will generate a random slot number between 0 and N-1 to
send a XID response frame. If the random slot number is 0
then it will transmit a reply immediately, otherwise it will wait
until it receives a XID command frame with a matching slot
number. Once the commander has sent out the N XID com- mand frames with slot numbers from 0 to N-1 it will send its
own details in a XID command frame with a slot number of
$FF.
The hint bits and nickname field are referred to as the XID
discovery   information   field   and   are   included   in   all   XID
response frames. For XID command frames only the final
frame with a slot number of $FF will contain this field.
The 32-bit device address is a random number that is cho-
sen at start up. It is possible that 2 devices will choose the
same   device   address   and   so   the   XID   frame   can   be
addressed to specific devices and can request that a new
32-bit address be generated.
The  IrDA  standard  recommends  that  if  a  XID  command
frame is received with a slot number greater that 0 while a
slot number has not been chosen (i.e. the slot 0 command
frame was missed) then a slot number should be chosen
between the received slot number and the maximum. This
complicates   generating   the   random   slot   number   and
requires registers to keep track of the XID process in case
more XID frames are missed. To avoid this the SX gener-
ates a random slot number every time a slot 0 XID com-
mand frame is detected and will send a XID response when
the slot number matches the chosen slot number. The SX
will respond correctly to a XID discovery as long as the first
frame of the discovery process is received correctly. If the
first frame is missed then the SX will not respond and the
discovery process will have to be repeated.
The random 32-bit device address is chosen at start-up and
will  be  re-generated  if  a  XID  command  frame  is  received
that  requests  a  new  address  be  generated.  Address  con-
flicts  can  only  detected  and  corrected  by  primary  IrDA
devices. The IrDA Virtual Peripheral is classed as a second-
ary  only  device  and  so  does  not  need  to  detect  conflicts.
The primary service of initiating XID discovery is only pro-
vided for information gathering of other devices in range. Payload TxEmpty Get data from payload layer (fl2plTxData).
Add to FCS. Last flag not set.
Jump to Send. Payload FCS Low TxEmpty Data = FCS Low XOR $FF.
Jump to Send FCS High FCS High TxEmpty Data = FCS High XOR $FF.
Jump to Send Send End Send End TxEmpty Send EOF byte Wait End Wait End TxEmpty Indicate complete to payload layer
(fl2plTxComplete) Idle (Send) Data <> BOF or EOF or CE Send data byte no change Data = BOF or EOF or CE Store current state.
Send CE byte Control Control TxEmpty Recover stored state.
Send data byte XOR $20 previous state Table 3-2.   Framing Layer Transmit State Machine Current State Event Action Next State