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Fast AMBA Peripherals
12-17
GMS30C7201 Data Sheet
12.3 Medium and Fast Infrared Module
12.3.1 Overview
The ARM 7201 Infrared Interface Module implements in hardware the physical layer of
an infrared serial port, compliant with Version 1.1 of the Infrared Data Association
(IrDA) standard. Communication speeds of up to 4Mbit/s are supported. When
combined with analogue transducer components, it provides a complete interface
between infra-red media and an AMBA-compliant peripheral bus (APB).
The module comprises three separate encoder/decoder units for implementing three
different combinations of modulation scheme and data encoding system defined by the
IrDA standard. These are called the Slow-, Medium- and Fast-infrared encoder/
decoders.
SIr, MIr and FIr
The Slow Encoder/Decoder (SIr) is used to modulate and demodulate serial data only,
using the Hewlett-Packard Serial Infrared standard (HP-SIR) for bit encoding. Serial
transmit data from a UART (external to the Ir Interface Module) is modulated using
return-to-zero (RTZ) encoding to produce an output to drive the IR transmitter LED,
while data received from the IR detector is converted into a serial bit stream to drive the
UART serial input. The SIr supports data rates of up to 115.2kbit/s.
The Medium Speed Encoder/Decoder (MIr) encodes/decodes peripheral bus data
according to a modified HDLC standard, using flag characters, bit-stuffing and a 16-bit
CRC checker. RTZ modulation and demodulation of the encoded data stream takes
place in the same way as for the SIr. Two signal bit rates are specified: 0.576Mbit/s and
1.152Mbit/s. MIr data and control bytes are memory mapped via the ARM 7201 fast
APB.
The Fast Speed Encoder/Decoder (FIr) operates at a fixed bit rate of 4Mbit/s.
Modulation/demodulation is by a phase shift key scheme called pulse position
modulation (4PPM) that uses one of four signalling symbols to represent each pair of
data bits. Data encoding uses a packet format that prefixes bit- and symbol-
synchronization flags to data and appends a 32-bit CRC and stop flag to the end of
each packet. The start and stop flags use signalling symbols that are not used to
encode data and hence bit-stuffing of data is not required in this mode. The FIr also
interfaces to the ARM 7201 fast APB.
Only one of the Encoder/Decoder modules can be enabled to transmit and receive data
from the IrDA transducers at one time. Selection of an Ir sub-module is by means of the
IrEnable register.
Accordingly the MIr and FIr sub-modules can be regarded by programmers as
independent entities which are operated using common control and data registers, but
which report status data via separate read registers.
Detailed descriptions of the MIr and FIr are given in the following sections. The SIr
however has no data or control register associated with it, and interfaces directly to the
UART serial streams. Accordingly with the exception of the Ir Enable register, it has no
presence on the memory map nor an interface to the APB.
12.3.2 Common Register Description
The Infrared Interface module uses a single mode register, IrEnable, to select which
infrared interface module is selected. The Medium and Fast modules share a common
control and data interface while maintaining separate status registers. The common
registers used by both the FIr and MIr blocks are described below.