DATA15
•
•
•
•
•
•
•
•
•
•
•
•
MODE SELECT
LINES
DATA1
DATA0
V
V
DD DD
*
*
*
RESET
DS
R/W
*Optional, to prevent conflict on RESET negation.
DATA BUS MODE DECODE
Figure 4-15 Data Bus Mode Select Conditioning
4
Data bus mode select current is specified in APPENDIX A ELECTRICAL CHARAC-
TERISTICS. Do not confuse pin function with pin electrical state. Refer to 4.6.5 Pin
State During Reset for more information.
DATA0 determines the function of the boot ROM chip-select signal (CSBOOT). Unlike
other chip-select signals, CSBOOT is active at the release of reset. During reset ex-
ception processing, the MCU fetches initialization vectors beginning at address
$000000 in supervisor program space. An external memory device containing vectors
located at these addresses can be enabled by CSBOOT after a reset. The logic level
of DATA0 during reset selects boot ROM port size for dynamic bus allocation. When
DATA0 is held low, port size is eight bits; when DATA0 is held high, either by the weak
internal pull-up driver or by an external pull-up, port size is 16 bits. Refer to 4.8.4 Chip-
Select Reset Operation for more information.
DATA1 and DATA2 determine the functions of CS[2:0] and CS[5:3], respectively. DA-
TA[7:3] determine the functions of an associated chip select and all lower-numbered
chip-selects down through CS6. For example, if DATA5 is pulled low during reset,
CS[8:6] are assigned alternate function as ADDR[21:19], and CS[10:9] remain chip-
selects. Refer to 4.8.4 Chip-Select Reset Operation for more information.
DATA8 determines the function of the DSACK[1:0], AVEC, DS, AS, and SIZE pins. If
DATA8 is held low during reset, these pins are assigned to I/O port E.
DATA9 determines the function of interrupt request pins IRQ[7:0] and the clock mode
select pin (MODCLK). When DATA9 is held low during reset, these pins are assigned
to I/O port F.
DATA11 determines whether the SIM operates in test mode out of reset. This capabil-
ity is used for factory testing of the MCU.
MC68331
SYSTEM INTEGRATION MODULE
MOTOROLA
4-39
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
