IRSCI Module Overview
12.3 IRSCI Module Overview
The IRSCI consists of a serial communications interface (SCI) and a infrared interface sub-module as
shown in Figure 12-2.
INTERNAL BUS
SCI_TxD
SCTxD
SERIAL
CGMXCLK
SCI_R32XCLK
SCI_R16XCLK
COMMUNICATIONS
INTERFACE MODULE
(SCI)
INFRARED
SUB-MODULE
BUS CLOCK
SCI_RxD
SCRxD
Figure 12-2. IRSCI Block Diagram
The SCI module provides serial data transmission and reception, with a programmable baud rate clock
based on the bus clock or the CGMXCLK.
The infrared sub-module receives two clock sources from the SCI module: SCI_R16XCLK and
SCI_R32XCLK. Both reference clocks are used to generate the narrow pulses during data transmission.
The SCI_R16XCLK and SCI_R32XCLK are internal clocks with frequencies that are 16 and 32 times the
baud rate respectively. Both SCI_R16XCLK and SCI_R32XCLK clocks are used for transmitting data.
The SCI_R16XCLK clock is used only for receiving data.
NOTE
For proper SCI function (transmit or receive), the bus clock MUST be
programmed to at least 32 times that of the selected baud rate.
When the infrared sub-module is disabled, signals on the TxD and RxD
pins pass through unchanged to the SCI module.
12.4 Infrared Functional Description
Figure 12-3 shows the structure of the infrared sub-module.
The infrared sub-module provides the capability of transmitting narrow pulses to an infrared LED and
receiving narrow pulses and transforming them to serial bits, which are sent to the SCI module. The
infrared sub-module receives two clocks from the SCI. One of these two clocks is selected as the base
clock to generate the 3/16, 1/16, or 1/32 bit width narrow pulses during transmission.
The sub-module consists of two main blocks: the transmit encoder and the receive decoder. When
transmitting data, the SCI data stream is encoded by the infrared sub-module. For every "0" bit, a narrow
"low" pulse is transmitted; no pulse is transmitted for "1" bits. When receiving data, the infrared pulses
should be detected using an infrared photo diode for conversion to CMOS voltage levels before
connecting to the RxD pin for the infrared decoder. The SCI data stream is reconstructed by stretching
the "0" pulses.
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor
183
FREESCALE [ Freescale ]
