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The RTS and CTS signal names follow EIA232 DTE conventions. Thus,
RTS is an input and CTS is an output. In many instances, these names
are reversed to reflect device drive direction. Check the specification of
the remote device to assure correct connection.
11.2.3 TXD — UART Transmit
This pin is the transmitter serial output. In normal mode, NRZ data is output. In infra-
red mode, a 3/16 bit-period pulse is output for each zero bit transmitted and no pulse
for each one bit transmitted. For EIA232 standard applications this pin must be con-
nected to an EIA232 transmitter to convert voltage levels. TXD can be programmed
as a general-purpose I/O pin when the UART TXD function is not being used.
11.2.4 RXD — UART Receive
This pin is the receiver serial input. In normal mode, NRZ data is expected. In infrared
mode, a narrow pulse is expected for each zero bit received and no pulse for a one bit
received. External circuitry must be used to convert the infrared signal to an electrical
signal. EIA232 standard applications require an external EIA232 receiver to convert
voltage levels. RXD can be programmed as a general-purpose I/O pin when the
UART RXD function is not being used.
11.3 Sub-Block Description
The UART contains four sub-modules. This section briefly describes the basic func-
tionality of the four blocks.
11.3.1 Transmitter
The transmitter accepts a parallel character from the CPU and transmits it serially.
The start, stop, and parity (if enabled) bits are added to the character. The transmitter
posts a maskable interrupt when it is ready for parallel data. RTS can be used to con-
trol the flow of the serial data. If RTS is negated (high), the transmitter finishes send-
ing the character in progress (if any) then stops and waits for RTS to again become
asserted (low).
A break character (continuous zeros) can be generated by the transmitter as well. For
debugging purposes, parity errors can be generated. The transmitter operates from
the 1x clock provided by the 16x bit clock generator.
Normal NRZ is transmitted when the infrared interface is disabled.
11.3.2 Receiver
The receiver accepts a serial data stream and converts it into a parallel character.
When enabled, it searches for a start bit, qualifies it, and then samples the succeed-
ing data bits at the bit-center. Jitter tolerance and noise immunity are provided by
sampling at a 16x rate and using voting techniques to clean up the samples. Once the
MMC2001
UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER MODULE
REFERENCE MANUAL
Freescale Semiconductor, Inc.
NOTE
For More Information On This Product,
Go to: www.freescale.com
MOTOROLA
11-3
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