LM185BYH-2.5 National Semiconductor, LM185BYH-2.5 Datasheet - Page 11

• Manufacturer Part Number: LM185BYH-2.5
• Description: Micropower Voltage Reference Diode
• Manufacturer: National Semiconductor

Capacitance
Additional Applications Information
POWERING UP
The ’ACT715/LM1882 default value for Bit 10 of the Status
Register is 0. This means that when the CLEAR pulse is ap-
plied 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 de-
fault 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 solu-
tion to automatically enable the CLOCK upon power-up. Use
of the ’ACT715-R/LM1882-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 af-
fected for the duration of one frame only. These signals are
the Horizontal and Vertical Drive signals. After a Vectored
Restart the beginning of these signals will occur at the first
CLK. The end of the signals will occur as programmed. At
the completion of the first frame, the signals will resume to
their programmed start and end time.
(Continued)
FIGURE 5. AC Specifications
11
PREPROGRAMMING “ON-THE-FLY”
Although the ’ACT715/LM1882 and ’ACT715-R/LM1882-R
are completely programmable, certain limitations must be
set as to when and how the parts can be reprogrammed.
Care must be taken when reprogramming any End Time reg-
isters to a new value that is lower than the current value.
Should the reprogramming 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 rec-
ommended when reprogramming “on-the-fly”. The first rec-
ommendation is to reprogram horizontal values during the
horizontal blank interval only and/or vertical values during
the vertical blank interval only. Since this would require deli-
cate timing requirements the second recommendation may
be more appropriate.
The second recommendation is to program a Vectored Re-
start 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 maintain
the video integrity.
DS100232-6
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