MC68HC916X1
MOTOROLA
MC68HC916X1TS/D
17
RWD — Read/Write Disable
0 = R/W signal operates normally
1 = R/W signal placed in high-impedance state.
RWD is cleared to zero when the MCU is in an expanded mode, and set to one in single-chip mode.
RWD can be written only once after reset.
IARB[3:0] — Interrupt Arbitration
Each module that can generate interrupts, including the SCIM, has an IARB field. Each IARB field can
be assigned a value from $0 to $F. During an interrupt acknowledge cycle, IARB permits arbitration
among simultaneous interrupts of the same priority level. The reset value of the SCIM IARB field is $F.
This prevents SCIM interrupts from being discarded. Initialization software must set the IARB field to a
lower value if lower priority interrupts are to be arbitrated.
3.2.1 Operating Modes
During reset, the SCIM configures itself according to the states of the DATA[11:0], BERR, MOD-
CLK, and BKPT pins. DATA[11:0] provide pin configuration information. BERR, MODCLK, and
BKPT determine basic operation.
The SCIM can be configured to operate in one of three modes: 16-bit expanded, 8-bit expanded,
and single chip. Operating mode is determined by the value of the DATA1 and BERR signals com-
ing out of reset. Table 9 shows the basic configuration options.
BERR, BKPT, and MODCLK do not have internal pull-ups and must be driven to the desired state
during reset.
Operating mode determines which address and data bus lines are used and which general-purpose
I/O ports are available. Table 10 is a summary of bus and port configuration options.
Many pins on the MC68HC916X1, including data and address bus pins, have multiple functions.
Reset value for these pins depends on operating mode. In expanded mode, the values of
DATA[11:0] during reset determine the function of these pins. The functions of some pins can be
changed by writing to the appropriate pin assignment register. Data bus pins have internal pull-ups
and must be pulled low to achieve the alternate configuration desired. The following tables contain
a summary of pin configuration options for each operating mode.
Table 9 Basic Configuration Options
Select Pin
Default Function
(Pin Left High)
Alternate Function
(Pin Pulled Low)
MODCLK
Synthesized System Clock
External System Clock
BKPT
Background Mode Disabled
Background Mode Enabled
BERR
Expanded Mode
Single-Chip Mode
DATA1 (if
BERR = 1)
8-Bit Expanded Mode
16-Bit Expanded Mode
Table 10 Bus and Port Configuration Options
Mode
Address Bus
Data Bus
I/O Ports
16-Bit Expanded
ADDR[18:3]
DATA[15:0]
—
8-Bit Expanded
ADDR[18:3]
DATA[15:8]
DATA[7:0] = Port H
Single Chip
None
ADDR[18:11] = Port A
ADDR[10:3] = Port B
DATA[15:8] = Port G
DATA[7:0] = Port H
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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