Cirrus Logic EP93xx - Manual (Page 609)

DS785UM1 17-13
Copyright 2007 Cirrus Logic
IrDA
EP93xx User’s Guide
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17.4.2.3 Transmit Operation
Immediately after enabling the MIR for transmission, the user may either “prime” the transmit
buffer by filling it with data (see section
Section 17.4.2 on page 17--11 for details) or allow
service requests to cause the CPU or DMA to fill the buffer once the MIR is enabled. Once
enabled, the transmit logic issues a service request if its buffer is empty. A Serial Infrared
Interaction Pulse (SIP) is transmitted in order to guarantee non-disruptive co-existence with
slower (up to 115.2 Kbps) systems, for example another device attempting to use its SIR.
This is followed by continuous transmission of flags until valid data resides within the buffer.
Once a byte of data resides at the bottom of the transmit buffer, it is transferred to the serial
shifter, is encoded and shifted out onto the transmit pin clocked by the programmed baud rate
clock. Note that the flags and CRC value are automatically transmitted and need not be
placed in the transmit buffer.
When the transmit buffer has space for another word, an interrupt and/or DMA service
request is signalled. If new data is not supplied soon enough, the buffer is completely
emptied and the transmit logic attempts to take additional data from the empty buffer, one of
two actions can be taken as programmed by the user. An underrun can either signal the
normal completion of a frame or an unexpected termination of a frame in progress.
When normal frame completion is selected and an underrun occurs, the transmit logic
transmits the 16 bit CRC value calculated during the transmission of all data within the frame
(including the address and control bytes), followed by a flag to denote the end of the frame.
The transmitter then transmits an SIP, followed by a continuous transmission of flags until
data is once again available within the buffer. Once data is available, the transmitter begins
transmission of the next frame.
When unexpected frame termination is selected and an underrun occurs, the transmit logic
outputs an abort and interrupts the CPU. An abort continues to be transmitted until data is
once again available in the transmit buffer. The MIR then transmits an SIP, followed by a
double flag and starts the new frame. The off-chip receiver may choose to ignore the abort
and continue to receive data, or to signal the serial port to retry transmission of the aborted
frame. If the user disables the transmitter during operation, transmission of the current data
byte is stopped immediately, the serial shifter and transmit buffer are cleared and all clocks
used by the transmit logic are automatically disabled to conserve power.
17.5 Fast IrDA Specific Features
The Fast Infrared port (FIR) operates at half-duplex and provides direct connection to
commercially available Infrared Data Association (IrDA) compliant LED transceivers. The FIR
supports the 4.0 Mbps IrDA standard, using four pulse position modulation (4 PPM) and a
specialized serial packet protocol developed expressly for IrDA transmission.