MC9S12HZ256 Datasheet, PDF (401/692 Pages) Freescale Semiconductor, Inc – HCS12 Microcontrollers

Chapter 13 Serial Communication Interface (SCIV4)
13.4.1 Infrared Interface Submodule
This module provides the capability of transmitting narrow pulses to an IR LED and receiving narrow
pulses and transforming them to serial bits, which are sent to the SCI. The IrDA physical layer
specification defines a half-duplex infrared communication link for exchange data. The full standard
includes data rates up to 16 Mbits/s. This design covers only data rates between 2.4 kbits/s and
115.2 kbits/s.
The infrared submodule consists of two major blocks: the transmit encoder and the receive decoder. The
SCI transmits serial bits of data which are encoded by the infrared submodule to transmit a narrow pulse
for every 0 bit. No pulse is transmitted for every 1 bit. When receiving data, the IR pulses should be
detected using an IR photo diode and transformed to CMOS levels by the IR receive decoder (external
from the MCU). The narrow pulses are then stretched by the infrared submodule to get back to a serial bit
stream to be received by the SCI. The polarity of transmitted pulses and expected receive pulses can be
inverted so that a direct connection can be made to external IrDA transceiver modules that uses active low
pulses.
The infrared submodule receives its clock sources from the SCI. One of these two clocks are selected in
the infrared submodule in order to generate either 3/16, 1/16, 1/32, or 1/4 narrow pulses during
transmission. The infrared block receives two clock sources from the SCI, R16XCLK, and R32XCLK,
which are configured to generate the narrow pulse width during transmission. The R16XCLK and
R32XCLK are internal clocks with frequencies 16 and 32 times the baud rate respectively. Both
R16XCLK and R32XCLK clocks are used for transmitting data. The receive decoder uses only the
R16XCLK clock.
13.4.1.1 Infrared Transmit Encoder
The infrared transmit encoder converts serial bits of data from transmit shift register to the TXD pin. A
narrow pulse is transmitted for a 0 bit and no pulse for a 1 bit. The narrow pulse is sent in the middle of
the bit with a duration of 1/32, 1/16, 3/16, or 1/4 of a bit time. A narrow high pulse is transmitted for a 0 bit
when TXPOL is cleared, while a narrow low pulse is transmitted for a 0 bit when TXPOL is set.
13.4.1.2 Infrared Receive Decoder
The infrared receive block converts data from the RXD pin to the receive shift register. A narrow pulse is
expected for each 0 received and no pulse is expected for each 1 received. A narrow high pulse is expected
for a 0 bit when RXPOL is cleared, while a narrow low pulse is expected for a 0 bit when RXPOL is set.
This receive decoder meets the edge jitter requirement as defined by the IrDA serial infrared physical layer
specification.
13.4.2 Data Format
The SCI uses the standard NRZ mark/space data format. When Infrared is enabled, the SCI uses RZI data
format where 0s are represented by light pulses and 1s remain low. See Figure 13-12.
MC9S12HZ256 Data Sheet, Rev. 2.04
Freescale Semiconductor
401