xr88c681 Exar Corporation, xr88c681 Datasheet - Page 36

• Manufacturer Part Number: xr88c681
• Description: Cmos Dual Channel Uart Duart
• Manufacturer: Exar Corporation

Figure 12 presents the block diagram of the MC68HC11 C.
The 68HC11 can be configured to operate in a “Single
Chip” Mode or in an “Expanded Multiplexed Bus” mode. If
a device is configured to operate in the “Single Chip”
mode, the entire 64K bytes of Address space is internal to
the C IC. Please note that this does not mean that there
is 64K bytes of memory, or other addressable portions
within the device. A 68HC11 MCU configured for “Single
Chip” mode operation cannot address any components
external to the MCU.
interface the DUART to this C, then the C must operate
in the “Expanded-Multiplexed” mode. The MC68HC11 is
Rev. 2.11
MODA
Mode Control
System
Port A
Timer
MODB
Therefore, if a user desired to
Figure 12. Block Diagram of the MC68HC11 Microcontroller
XTAL EXTAL
Oscillator
Clock Logic
Bus Expansion
Strobe and Handshake Parallel I/O
Address
Port B
8
E
CPU Core
-IRQ -XIRQ -RESET
Interrupt
Address/Data
Logic
36
Control
Port C
configured into the Expanded Multiplexed mode by tying
both the MODA and MODB pin to Vcc, and then resetting
the device.
The MC68HC11 consists of 5 different multi-function
parallel ports. Each of these ports are briefly discussed.
Port A
Port A consists of 3 input pins, 4 output pins and 1
bi-directional pin. This port is used to support the Timer
System. One of the input pins can be used for the Pulse
SPI
Control
Port D
RAM 256 Bytes
SCI
A/D Converter
Port E
VRH
VRL