P R E L I M I N A R Y
PULSE WIDTH DEMODULATION
ASYNCHRONOUS SERIAL PORTS
For many applications, such as bar-code reading, it is
necessary to measure the width of a signal in both its
High and Low phases. The Am186ES and Am188ES
microcontrollers provide a pulse-width demodulation
(PWD) option to fulfill this need. The PWD bit in the
system configuration register (SYSCON) enables the
PWD option. Please note that the Am186ES and
Am188ES microcontrollers do not support analog-to-
digital conversion.
The Am186ES and Am188ES microcontrollers provide
two independent asynchronous serial ports. These
ports provide full-duplex, bidirectional data transfer
using several industry-standard communications pro-
tocols. The serial ports can be used as sources or des-
tinations of DMA transfers.
The asynchronous serial ports support the following
features:
n Full-duplex operation
n 7-bit, 8-bit, or 9-bit data transfers
n Odd, even, or no parity
n One stop bit
In PWD mode, TMRIN0, TMRIN1, INT2, and INT4 are
configured internal to the microcontroller to support the
detection of rising and falling edges on the PWD input
pin (INT2/INTA0/PWD) and to enable either timer 0
when the signal is High or timer 1 when the signal is
Low. The INT4, TMRIN0, and TMRIN1 pins are not
used in PWD mode and so are available for use as
PIOs.
n Two lengths of break characters
n Error detection
— Parity errors
The following diagram shows the behavior of a system
for a typical waveform.
— Framing errors
— Overrun errors
n Hardware handshaking with the following select-
able control signals:
— Clear-to-send (CTS)
INT4
INT2 Ints generated
TMR1 enabled
— Enable-receiver-request (ENRX)
— Ready-to-send (RTS)
INT2
TMR0 enabled
— Ready-to-receive (RTR)
n DMA to and from the serial ports
The interrupt service routine (ISR) for the INT2 and
INT4 interrupts should examine the current count of the
associated timer, timer 1 for INT2 and timer 0 for INT4,
in order to determine the pulse width. The ISR should
then reset the timer count register in preparation for the
next pulse.
n Separate maskable interrupts for each port
n Multidrop protocol (9-bit) support
n Independent baud rate generators
n Maximum baud rate of 1/16th of the CPU clock
n Double-buffered transmit and receive
Since the timers count at one quarter of the processor
clock rate, this determines the maximum resolution that
can be obtained. Further, in applications where the
pulse width may be short, it may be necessary to poll
the INT2 and INT4 request bits in the interrupt request
register in order to avoid the overhead involved in tak-
ing and returning from an interrupt. Overflow condi-
tions, where the pulse width is greater than the
maximum count of the timer, can be detected by mon-
itoring the Maximum Count (MC) bit in the associated
timer or by setting the INT bit to enable timer interrupt
requests.
DMA Transfers through the Serial Port
The Am186ES and Am188ES microcontrollers support
DMA transfers both to and from the serial port. Either
or both DMA channels and either or both serial ports
can be used for DMA transmits or receives. See the
DMA Control register descriptions in the Am186ES and
Am188ES Microcontrollers User’s Manual for more in-
formation.
Am186/188ES and Am186/188ESLV Microcontrollers
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