Fast AMBA Peripherals
12-32
GMS30C7201 Data Sheet
12.8 Medium Speed Infra-red Port (MIr)
The MIr comprises a dedicated serial port and RTZ modulator/demodulator supporting
the Infrared Data Association (IrDA) standard for transmission/reception at 0.576 and
1.152Mb/s.
Frames contain an 8-bit address, an optional control field, a data field of any size that
is a multiple of 8-bits and a 16-bit CRC-CCITT. The start/stop flag and CRC generation/
checking is handled automatically. Data can be selectively saved in the receive buffer
by programming an address with which to compare against all incoming frames.
Interrupts are signalled when CRC checks performed on received data indicate an
error, when a receiver abort occurs, when the transmit buffer underruns during an
active frame and is aborted, when the receive buffer overruns and data is lost.
12.8.1 MIr (Medium-speed Infrared) Operation
Following reset, the MIr is disabled. Reset also causes the transmit and receive buffers
and tail register to be flushed (buffers marked as empty). Before enabling the MIr, the
user must first clear any writable or
“
sticky
”
status bits that are set by writing a one to
each bit. (A sticky bit is a readable status bit that may be cleared by writing a one to its
location.) Next, the desired mode of operation is programmed in the control register. At
this point the user may
“
prime
”
the transmit buffer by writing the first data word for
transmission and any tail bytes, or the buffer can remain empty and either programmed
I/O or the DMA may be used to service it after the MIr is enabled. Once the MIr is
enabled, transmission/reception of data can begin on the transmit and receive pins.
Bit Encoding
The MIr bit encoding uses an RZI modulation scheme where a
‘
0
’
is represented by a
light pulse. For both 0.576 and 1.152Mb/s data rates, the optical pulse duration is
normally
1
/
of a bit duration. For example, if the data frame (in the order of transmission)
is 11010010, then
Figure 12-7: RZ1/NRZ bit encoding example
represents the signal
that is actually transmitted.
Figure 12-7: RZ1/NRZ bit encoding example
Frame Format
MIr uses a flag (reserved bit pattern) to denote the beginning and end of a frame of
information and to synchronize frame transmission. A double flag is used to indicate the
start of a frame, and a single flag the end. The flag contains eight bits, which start and
end with a zero and contain six sequential ones in the middle (01111110). This
sequence of six ones is unique because all data between the start and stop flag is
prohibited from having more than five consecutive ones. Data that violates this rule is
altered before transmission by automatically inserting a zero after five consecutive
ones are detected in the transmitted bit stream. This technique is commonly referred to
NRZ
Data
Bit
Value
1
1
0
1
0
0
1
0
LSB
MSB
RZ1