Infrared Serial Communications Interface Module (IRSCI)
TNP[1:0]
IREN
IR_TxD
TRANSMIT
SCI_TxD
ENCODER
SCTxD
MUX
SCI_R32XCLK
SCI_R16XCLK
IR_RxD
RECEIVE
SCRxD
DECODER
SCI_RxD
MUX
Figure 12-3. Infrared Sub-Module Diagram
12.4.1 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 12-4. Infrared SCI Data Example
12.4.2 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".
MC68HC908AP Family Data Sheet, Rev. 4
184
Freescale Semiconductor
FREESCALE [ Freescale ]
