BeagleBoard XM C2, Справочное руководство

Страница: 102 / 163

REF: BB_SRM_xM
BeagleBoard-xM System
Reference Manual
Revision C2
Page 102 of 163
All of the I/O levels from the processor are 1.8V while the transceiver used runs at 3.3V.
This requires that the voltage levels be translated. This is accomplished by the TXS0102
which is a two-bit noninverting translator that uses two separate configurable power-
supply rails. The A port tracks VCCA, 1.8V and the B port tracks VCCB, 3.3V. This
allows for low-voltage bidirectional translation between the two voltage nodes. When the
output-enable (OE) input is low, all outputs are placed in the high-impedance state. In
this design, the OE is tied high via a 10K ohm resistor to insure that it is always on.
7.21.3 RS232 Transceiver
The RS232 transceiver used is the
SN65C322 which consists of one line driver, one line
receiver, and a dual charge-pump circuit with ±15-kV IEC ESD protection pin to pin
(serial-port connection pins, including GND). These devices provide the electrical
interface between an asynchronous communication controller and the serial-port
connector. The charge pump and four small external capacitors allow operation from a
single 3-V to 5.5-V supply. The
SN65C3221 operates at data signaling rates up to 1
Mbit/s and a driver output slew rate of 24 V/ms to 150 V/ms. While the processor can
easily drive a 1Mbit/S rate, your results may vary based on cabling, distance, and the
loads and drive capability on the other end of the RS232 port.
The transceiver is powered from the 3.3V rail and is active at power up. This allows the
port to be used for UART based peripheral booting over the port.
7.21.4 Connector
Access to the RS232 port is through a 9 pin DB9 connector,
P9 . This is new on the –xM
version and replaces the 10 pin header. A standard male to female straight DB9 cable can
be used or a USB to DB9 adapter can be plugged direct into the board.
7.22 Indicators
There are five green indicators on the BeagleBoard:
o
Power
o
PMU_STAT
o
USER0
o
USER1
o
HUB Power
All of the green LEDs are programmable under software control. Figure 49 shows the
connection of all of these indicators.