MMC2001
REFERENCE MANUAL
UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER MODULE
MOTOROLA
11-3
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 RS-232 applications this pin must be connected to
an RS-232 transmitter. 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 elec-
trical signal. RS-232 applications need an external RS-232 receiver to convert volt-
age 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 start bit has been found, the data bits, parity bit (if parity is enabled), and stop bits
are shifted in. If parity is enabled, it is checked and its status is reported in the RX
register. Similarly, frame errors and breaks are checked and reported. When a new
character is ready to be read by the host, RX READY is asserted and an interrupt is
posted (if enabled). If the receiver register is read as a 16-bit word, the interrupt is
automatically cleared and the data, along with four status bits, are read by the CPU.
CTS can be configured as an output to indicate a pending overrun.
Normal NRZ is expected when the infrared interface is disabled.