
5-30
M68000 8-/16-/32-BIT MICROPROCESSORS USER'S MANUAL
MOTOROLA
executing a reset instruction is ignored. Since the processor asserts the 
RESET
 signal for
124 clock cycles during execution of a reset instruction, an external reset should assert
RESET
 for at least 132 clock periods.
5.6 THE RELATIONSHIP OF 
DTACK
, 
BERR
, AND  
HALT
To properly control termination of a bus cycle for a retry or a bus error condition, 
DTACK
,
BERR
, and 
HALT
 should be asserted and negated on the rising edge of the processor
clock. This relationship assures that when two signals are asserted simultaneously, the
required setup time (specification #47, 
Section 9 Electrical Characteristics
) for both of
them is met during the same bus state. External circuitry should be designed to
incorporate this precaution. A related specification, #48, can be ignored when 
DTACK
,
BERR
, and 
HALT
 are asserted and negated on the rising edge of the processor clock.
The possible bus cycle termination can be summarized as follows (case numbers refer to
Table 5-5).
Normal Termination:
DTACK
 is asserted. 
BERR
 and 
HALT
 remain negated (case 1).
Halt Termination:
HALT
 is asserted coincident with or preceding 
DTACK
, and
BERR
 remains negated (case 2).
Bus Error Termination:
BERR
 is asserted in lieu of, coincident with, or preceding
DTACK
 (case 3). In the MC68010, the late bus error also,
BERR
 is asserted following 
DTACK
 (case 4). 
HALT
 remains
negated and 
BERR
 is negated coincident with or after 
DTACK
.
Retry Termination:
HALT
 and 
BERR
 asserted in lieu of, coincident with, or before
DTACK
 (case 5). In the MC68010, the late retry also, 
BERR
and 
HALT
 are asserted following 
DTACK
 (case 6). 
BERR
 is
negated coincident with or after 
DTACK
. 
HALT
 must be held at
least one cycle after 
BERR
.
Table 5-1 shows the details of the resulting bus cycle termination in the M68000
microprocessors for various combinations of signal sequences.