MC68000 MOTOROLA [Motorola, Inc], MC68000 Datasheet - Page 97

• Manufacturer Part Number: MC68000
• Manufacturer: MOTOROLA [Motorola, Inc]

EN—Enable
3.6.2.2 Option Registers (OR3–OR0)
These four 16-bit registers consist of a base address mask field, a read/write mask bit, a
compare function code bit, and a DTACK generation field.
Bits 15–12—DTACK Field
When all the bits in this field are set to one, DTACK must be generated externally, and the
IMP or external bus master waits for DTACK (input) to terminate its bus cycle. After system
reset, the bits of the DTACK field default to six wait states.
The DTACK generator uses the IMP internal clock to generate the programmable number
of wait states. For asynchronous external bus masters, the programmable number of wait
states is counted directly from the internal clock. When no wait state is programmed
(DTACK = 000), the DTACK generator will generate DTACK asynchronously.
The CS lines are asserted slightly earlier for internal IMP master memory cycles than for an
external master using the CS lines. Set external master wait state (EMWS) in the SCR
whenever these timing differences require an extra memory wait state for external masters.
MOTOROLA
After system reset, only CS0 is enabled; CS3–CS1 are disabled. In disable CPU mode,
CS3–CS0 are disabled at system reset. The chip select does not require disabling before
changing its parameters.
These bits are used to determine whether DTACK is generated internally with a program-
mable number of wait states or externally by the peripheral. With internal DTACK gener-
ation, zero to six wait states can be automatically inserted before the DTACK pin is
asserted as an output (see Port A Control Register (PACNT)).
15
0 = The chip-select line is disabled.
1 = The chip-select line is enabled.
DTACK
13
12
15
0
0
0
0
1
1
1
1
Table 3-8. DTACK Field Encoding
Bits
14
MC68302 USER’S MANUAL
0
0
1
1
0
0
1
1
BASE ADDRESS MASK (M23–M13)
13
0
1
0
1
0
1
0
1
No Wait State
1 Wait State
2 Wait States
3 Wait States
4 Wait States
5 Wait States
6 Wait States
External DTACK
Description
System Integration Block (SIB)
2
MRW
1
CFC
3-47
0