MC68HC908LJ24 Datasheet, PDF (251/464 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Infrared Serial Communications Interface Module (IRSCI)
Infrared Functional Description
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.
13.6.1 Infrared Transmit Encoder
The infrared transmit encoder converts the "0" bits in the serial data
stream from the SCI module to narrow "low" pulses, to the TxD pin. The
narrow pulse is sent with a duration of 1/32, 1/16, or 3/16 of a data bit
width. When two consecutive zeros are sent, the two consecutive narrow
pulses will be separated by a time equal to a data bit width.
DATA BIT WIDTH DETERMINED BY BAUD RATE
SCI DATA
INFRARED
SCI DATA
PULSE WIDTH = 1/32, 1/16, OR 3/16 DATA BIT WIDTH
Figure 13-4. Infrared SCI Data Example
13.6.2 Infrared Receive Decoder
The infrared receive decoder converts low narrow pulses from the RxD
pin to standard SCI data bits. The reference clock, SCI_R16XCLK,
clocks a four bit internal counter which counts from 0 to 15. An incoming
pulse starts the internal counter and a "0" is sent out to the IR_RxD
output. Subsequent incoming pulses are ignored when the counter count
is between 0 and 7; IR_RxD remains "0". Once the counter passes 7, an
incoming pulse will reset the counter; IR_RxD remains "0". When the
counter reaches 15, the IR_RxD output returns to "1", the counter stops
and waits for further pulses. A pulse is interpreted as jitter if it arrives
shortly after the counter reaches 15; IR_RxD remains "1".
MC68HC908LJ24/LK24 — Rev. 2
MOTOROLA
Infrared Serial Communications Interface Module (IRSCI)
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Data Sheet
251