
MOTOROLA
11-4
UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER MODULE
MMC2001
REFERENCE MANUAL
11.3.3 Infrared Interface
The infrared interface converts data to be transmitted or received as specified in the
IRDA Serial Infrared Physical Layer Specification.
For each zero to be transmitted, a narrow pulse which is 3/16 of a bit time is gener-
ated. For each one to be transmitted, no pulse is generated. External circuitry must
be provided to drive an infrared LED.
When the UART is receiving data, a narrow pulse is expected for each zero transmit-
ted, and no pulse is expected for each one transmitted. Circuitry external to the IC
transforms the infrared signal to an electrical signal.
11.3.4 16x Bit Clock Generator
The 16x bit clock generator provides the pre-scaled bit clocks to the transmitter and
receiver blocks. A divide ratio from one to 4096 may be selected in the UBRGR regis-
ter. The 16x bit clock generator provides sufficient flexibility to provide almost any
“standard” bit-clock from a variety of clock frequencies.
NOTE
The baud rate error is computed as follows for 115.2 Kbps:
The input clock is
The divide ratio selected is
Actual baud rate:
Actual-required rates ratio:
This results in an error-per-bit ratio of 1.27%.
16.38 Mhz
9 (UBRGR[11:0] = 8)
16.38 Mhz/9/16 = 113.75 khz
115.2/113.75 = 1.0127
11.3.5 General UART Definitions
The following definitions apply to both the transmitter and receiver operation:
Bit Time
— The time required to serially transmit or receive one bit of data.
Start Bit
— One bit time of logic zero that indicates the beginning of a data frame. A
start bit must begin with a one-to-zero transition.
Stop Bit
— One bit time of logic one that indicates the end of a data frame.
Frame
— A start bit, followed by a specified number of data or information bits, termi-
nated by one or two stop bits. The number of data or information bits must agree
between the transmitting and receiving devices. The most common frame format is
one start bit followed by eight data bits (LSB first) terminated by one stop bit, for a
total of ten bit times in the frame. The UART optionally provides other data formats as
specified through the control registers.
Break
— A frame in which all the bits are logic zero. This includes the stop bit, which
is normally a logic one, as well as the data bits. This kind of a frame is generally sent
to signal the end of a message or the beginning of a new message.