LM8400 National Semiconductor, LM8400 Datasheet - Page 15

• Manufacturer Part Number: LM8400
• Description: Four Wire Resistive Touchscreen Controller with Brownout
• Manufacturer: National Semiconductor

8.0 Brownout Reset
If the Brownout Reset feature is enabled, the internal reset
will not be turned off until t
internal reset will perform the same functions as external
reset. The device is guaranteed to operate at the specified
frequency down to the specified brownout voltage. After the
underflow, the logic is designed such that no additional
internal resets occur as long as V
brownout voltage.
The device is relatively immune to short duration negative
going V
power supply voltage is essential for proper brownout func-
tionality. Power supply decoupling is vital even in battery
powered systems.
There are two optional brownout voltages. The part numbers
for the three versions of this device are:
LM8500, V
LM8300, V
LM8400, BOR is disabled.
Refer to the device specifications for the actual V
ages.
High brownout voltage devices are guaranteed to operate at
10MHz down to the high brownout voltage. Low brownout
voltage devices are guaranteed to operate at 3.33MHz down
to the low brownout voltage. Low brownout voltage devices
are not guaranteed to operate at 10MHz down to the low
brownout voltage.
Under no circumstances should the RESET pin be allowed
to float. If the on chip Brownout Reset feature is being used,
the RESET pin should be connected directly to V
CC
bor
bor
transients (glitches). Proper filtering of the V
= high voltage range.
= low voltage range.
id
counts down to zero. The
(Continued)
CC
remains above the
FIGURE 4. Brownout Reset Operation
CC
bor
. The
volt-
CC
15
RESET input may also be connected to an external pull-up
resistor or to other external circuitry. The output of the brown-
out reset detector will always preset t
120-128µs for the LM8500 and 360-384µs for the LM8300.
At this time, the internal reset will be generated.
If the BOR feature is disabled, then no internal resets are
generated. In this case, the external RESET must be used.
When BOR is disabled, this on-chip circuitry is disabled and
draws no DC current.
9.0 Calibration
The device supports two, five, and thirteen point calibration.
If desired, the calibration points can be stored internally in
the non-volatile storage element. During calibration time, the
device has the unique ability to check if the calibration points
make sense. For instance, if the desired calibration point
was to be at the upper left hand corner, but by accident, the
actual point detected was at the lower right hand corner, the
device will reject this point. This ensures the calibration
process is not performed incorrectly.
The calibration checking, can be enabled or disabled. If the
calibration checking is enabled, the controller will always
check the current calibration point being done against the
theoretical point and determine if the value is within
the raw A/D values. If this option is desired, the touch screen
driver must perform the calibration according to the calibra-
tion mapping for the two, five, and thirteen points as shown
in Figure 5, Figure 6, and Figure 7, respectively. The theo-
retical values for the two, five, and thirteen points are shown
in Table 4, Table 5, and Table 6 respectively.
id
to a value between
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