74ACT715 Ver la hoja de datos (PDF) - Fairchild Semiconductor
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74ACT715 Datasheet PDF : 14 Pages
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Capacitance (Continued)
FIGURE 5. AC Specifications
Additional Applications Information
POWERING UP
The ACT715 default value for Bit 10 of the Status Register
is 0. This means that when the CLEAR pulse is applied and
the registers are initialized by loading the default values the
CLOCK is disabled. Before operation can begin, Bit 10
must be changed to a 1 to enable CLOCK. If the default
values are needed (no other programming is required) then
Figure 6 illustrates a hardwired solution to facilitate the
enabling of the CLOCK after power-up. Should control sig-
nals be difficult to obtain, Figure 7 illustrates a possible
solution to automatically enable the CLOCK upon power-
up. Use of the ACT715-R eliminates the need for most of
this circuitry. Modifications of the Figure 7 circuit can be
made to obtain the lone CLEAR pulse still needed upon
power-up.
Note that, although during a Vectored Restart none of the
preprogrammed registers are affected, some signals are
affected for the duration of one frame only. These signals
are the Horizontal and Vertical Drive signals. After a Vec-
tored Restart the beginning of these signals will occur at
the first CLK. The end of the signals will occur as pro-
grammed. At the completion of the first frame, the signals
will resume to their programmed start and end time.
PREPROGRAMMING “ON-THE-FLY”
Although the ACT715 and ACT715-R are completely pro-
grammable, certain limitations must be set as to when and
how the parts can be reprogrammed. Care must be taken
when reprogramming any End Time registers to a new
value that is lower than the current value. Should the repro-
gramming occur when the counters are at a count after the
new value but before the old value, then the counters will
continue to count up to 4096 before rolling over.
For this reason one of the following two precautions are
recommended when reprogramming “on-the-fly”. The first
recommendation is to reprogram horizontal values during
the horizontal blank interval only and/or vertical values dur-
ing the vertical blank interval only. Since this would require
delicate timing requirements the second recommendation
may be more appropriate.
The second recommendation is to program a Vectored
Restart as the final step of reprogramming. This will ensure
that all registers are set to the newly programmed values
and that all counters restart at the first CLK position. This
will avoid overrunning the counter end times and will main-
tain the video integrity.
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