IP2022 User’s Manual—Peripherals www.ubicom.com 273 version of the transmit output that is resistively combined outside
the chip before driving the magnetics. The data encode block performs polarity inversion, if necessary,
then in 10Base-T mode it performs Manchester encoding. In USB
bus  mode,  it  performs  bit  stuffing  and  then  NRZI  encoding.  Bit
stuffing  means  that  after  six  consecutive  ones,  a  zero  bit  is
inserted.  The  active  low  SxOE  pin  is  used  to  enable  the  USB
transceiver for transmission. Otherwise, this pin is held high. For
10Base-T, the output pins of the serializer are driven low when not
transmitting. The encode block is bypassed for all other protocols. For  transmitting,  software  must  specify  the  number  of  bits  to
transmit and load the data in the SxTBUF register. This data is
then  transferred  to  an  internal  register,  from  which  it  is  serially
shifted out to the transmit logic. The TXBE bit in the INTE register
can  be  set  to  enable  an  interrupt  when  the  data  has  been
transferred from the SxTBUF register. When there is a transmit
buffer underrun event (i.e. all of the data has been shifted out from
the  internal  register,  but  the  SxTBUF  register  has  not  been
reloaded),  an  EOP  condition  is  generated  on  the  SxTXP  and
SxTXM outputs after an internal counter decrements to zero. The
TXEOP bit in the SxINTE register can be set to enable an interrupt
when an underrun event occurs. For protocols other than USB and
Ethernet, the EOP generator is bypassed.