© 2000 Ubicom, Inc. All rights reserved. - 24 - www.ubicom.com AN16 SX IrDA Virtual Peripheral Implementation 6.0 Hardware The hardware consists of the following main aspects:
•    Ubicom SX communications controller
•    IR transceiver interface
•    RS232 serial interface
•    Port C break-out interface
•    Indication LEDs
•    Power supply
The IR transceiver circuit is based on the HP HSDL1001
transceiver.  The  current  limiting  resistor  for  the  trans-
ceiver LED was chosen to maximize the LED current and
thus  the  effective  range  of  the  link  while  operating  at
115200bps. The LED is capable of handling an average
current  of  100mA  and  a  peak  current  of  1000mA.  The
pulse will always be 2.16us (108 clock cycles @ 50MHz)
in length regardless of the baud-rate and the minimum bit
time   is   8.64us   (115200bps   =   4*108   clock   cycles   @
50MHz). The worst case character ‘0’ consists of 1 start
pulse,  8  data  pulses,  and  one  stop  no-pulse  thus  the
worst case average current percentage of peak current is
22.5%  (2.16us  /  8.64us  *  9/10bits).  The  voltage  drop
across the LED varies with the LED current and as such
the resistor value is hard to calculate for a desired current
greater  than  the  typical  application  information  given  in
the  data  sheet.  It  was  chosen  to  use  four  22W  0.125W
resistors in parallel to give an effective resistance of 5.5W
with a maximum power dissipation of 0.125W. In one of
the prototypes the peak LED current was measured to be
454mA  (2.52V  /  5.556W)  thus  making  the  worst-case
average  current  102.15mA  (454mA  *  22.5%)  and  the
worst-case   average   power   dissipation   of   257.42mW
(2.52V * 102.15mA). The LED current is acceptable how-
ever the current will be susceptible to component toler-
ances   and   care   must   be   taken.   The   resistor   power
dissipation  is  not  a  cause  for  concern  due  to  the  half-
duplex   nature   of   the   communications   reducing   the
longer-term average power dissipation. It should also be noted that the IR transceiver LED timing is completely software controlled. If the SX device leaves the LED on continually then the IR transceiver may be permanently damaged as it will be operating at 4.5 times its absolute maximum aver-
age current rating. The  RS232  interface  provides  a  115200bps  full-duplex
serial interface with RTS/CTS hardware hand-shaking. It
is based on a MAX232 compatible charge-pump voltage
converter to provide the voltage conversion and is con-
troller by bits 0 to 3 of port B.
The port C break-out interface provides access to port C
of  the  SX  device  (8-bit  bi-directional)  and  well  as  the
power supply rails.
The power supply consists of a bridge rectifier and a lin-
ear voltage regulator to allow the circuit to operate of a
range of power sources. The regulator has a typical drop-
out voltage of 2V thus for a DC source the voltage should
be  greater  than  8.4V  (5V  rail  +  2V  regulator  +  1.4V
bridge).
The  power  supply  de-coupling  capacitors  are  of  high
importance as the HP transceiver is highly susceptible to
power  supply  noise.  With  insufficient  power  supply  de-
coupling  the  transceiver  will  give  falsely  indicate  that  a
pulse has been received when the RS232 charge pump
spikes the power supply rail. The transceiver is protected
by  a  150uF  low  ESR  tantalum  capacitor  (C2)  and  a
100nF  X7R  ceramic  capacitor  (C9).  C2  is  shown  in  the
circuit diagram as being part of the power supply circuit
however  it  is  intended  primarily  to  protect  the  IR  trans-
ceiver and must be placed as close as practicable to the
transceiver.
The  port  C  breakout  has  the  following  pinout  (looking
down on the pins) where the +5V pin is pin 1: GND C6 C5 C3 C2 C1 C0 +5V C4 C7