AM186ES Advanced Micro Devices, AM186ES Datasheet - Page 154

• Manufacturer Part Number: AM186ES
• Description: microcontrollers provide a low-cost/ high-performance solution for embedded system designers who wish to use the x86 architecture.
• Manufacturer: Advanced Micro Devices

bit reset (Low) is ignored. The transmit portion of the port behaves identically with mode 3
operation.
This mode can be used in conjunction with mode 3 to allow for multidrop communications
over a common serial link. In this case, the serial port is configured as mode 2 initially. Each
time data is received with the ninth bit set, the data is compared by software against a
unique ID for this receiver. If the received data does not match the port ID, the port is left
in mode 2. If the received data matches the port ID, software should reconfigure the serial
port to mode 3, allowing it to receive 9-bit data with the ninth bit reset.
In a serial multidrop configuration, multiple serial ports are attached to the same serial line.
The master serial port is configured in mode 3 while the slave serial ports are configured
in mode 2. The master polls the other devices by sending out status request packets. Each
of these status request packets begins with an address byte (i.e. ninth data bit is set). The
slave ports report a receive character for the address byte since the ninth bit is set. Each
port then attempts to match the address against its own address. If the addresses do not
match, the port remains in mode 2 and ignores the remainder of the message. If the
addresses match, software reconfigures the port into mode 3. The two mode 3 ports are
able to exchange data freely.
It should be noted that only ports which are actively exchanging data (i.e. ports in mode 3)
should have hardware handshaking enabled. If this is not the case, multiple devices may
be driving the hardware handshaking lines. For this reason, hardware handshaking is not
supported for the mode 2 configuration and should not be enabled. In addition, if it is possible
for more than two devices to be configured as mode 3 at any one time, hardware
handshaking should not be enabled.
Mode 3 supports 8 data bits when parity is enabled or 9 data bits with parity disabled. When
not using parity, the ninth bit (bit 8) for transmission is set by writing a 1 to the TB8 field in
the serial port control register. The ninth data bit for a receive can be read in the RB8 field
of the serial port status register. See the discussion of the TB8 and RB8 fields for more
information.
This mode can be used in conjunction with mode 2 (see above) to allow for multidrop
communications over a common serial link. In this case, parity must be disabled. In this
configuration, software interprets receive characters as data as long as the ninth data bit
is reset (Low). When a character is received with the ninth bit set, software should compare
the lower eight bits against the port ID. If the port ID matches the receive data, the port
should remain in mode 3. If the port ID does not match the receive data, the port should
be reconfigured to mode 2.
Mode 4—In this mode, each frame consists of 7 data bits, a start bit, and a stop bit. Parity
is not available in this mode.
10-8
Asynchronous Serial Port