System Integration Module
MOTOROLA
MC68307 USER’S MANUAL
5-23
BA23–BA12—Base Addresses
This field should be initialized to provide the base address for the block of on-chip periph-
eral registers. As only address bits from A12 upwards can be specified, the block of on-
chip locations thus resides at an address which is a multiple of 4096. This block can be
anywhere in the address space, although should not overlap with the other chip selected
areas required in the user's application.
5.2.1.2 SYSTEM CONTROL REGISTER (SCR). The SCR can be read or written at any
time by 8-bit, 16-bit or 32-bit transfers. It is a 32-bit read/write location, and resides in the
exception vector table, at address hex $0000F4–$0000F7, in supervisor data space. The
SCR cannot be accessed in user data space. The register consists of eight status bits (bits
31–24, also called the system status register), and 24 system control bits. The eight system
status bits are normally 0 and are set to 1 by some event in the system. Writing a 0 to these
locations has no effect; writing a 1 clears the status bit.
The meanings of the various bits within this register are described in Section 5.2.1.3 5.2.1.3 SYSTEM STATUS REGISTER BITS DESCRIPTION. The
following
paragraphs
describe the system status register bits.
ADC—Address Decode Conflict
This bit is set when a conflict has occurred in the chip select logic because two or more
chip select lines attempt assertion in the same bus cycle. This conflict may be caused by
a programming error in which the user-allocated memory areas for each chip select over-
lap each other. Provided the ADCE bit is set, if ADC is set this causes BERR to be assert-
ed. If this bit is already set when another address decode conflict occurs, BERR is still
generated. The chip select logic protects the MC68307 from issuing two simultaneous
chip selects by employing a priority system. Write a one to this location to clear the status
bit. Writing a zero has no effect.
SCR
$0000F4
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
ADC
WPV
HWT
—
RS1–0
—
ADCE
WPVE
EPCS
E8051
BUSW0 BUSW1 BUSW2 BUSW3
RESET:
0
X
0
X
0
BUSW
1
Read/Write
Supervisor only
SCR continued
$0000F6
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
HWDE
HW2–0
UACW
UACD
TMCD
MBCD
LPEN
CDEN
CKD
EBUSW
—
CD2–0
RESET:
1
0
1
0
Read/Write
Supervisor only