LM8400 National Semiconductor, LM8400 Datasheet - Page 14

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

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7.0 Averaging Algorithm
7.3 Communication Modes
Liftoff
When set to the liftoff mode, the controller only sends data
through the UART on liftoff. The controller continuously
samples the touchscreen as long as there is a touch de-
tected on the touchscreen but only the last coordinate is sent
out to the UART.
Touchdown
When set to the touchdown mode, the controller only sends
data through the UART on touchdown. The controller con-
tinuously samples the touchscreen as long as there is a
touch detected on the touchscreen but only the first coordi-
nate is sent out to the UART.
Streaming
When set to the streaming mode, the controller continuously
sends data through the UART as long as there is a touch
detected on the touchscreen.
8.0 Brownout Reset
The device is initialized when the RESET pin is pulled low or
the On-chip Brownout Reset is activated. The Brownout
Reset feature is not available on the LM8400.
The RESET input initializes the device when pulled low. The
RESET pin must be held low for a minimum of 0.5µs for the
LM8500 and a minimum of 1.5µs for the LM8300/8400 to
guarantee a valid reset. During Power-Up initialization, the
user must ensure that the RESET pin of a device without the
Brownout Reset feature is held low until the device is within
the specified V
with a delay 5 times (5x) greater than the power supply rise
time is recommended. Reset should also be wide enough to
ensure crystal start-up upon power-up. A recommended re-
set circuit for this device is shown in Figure 3.
CC
voltage. An R/C circuit on the RESET pin
(Continued)
14
When enabled, the device generates an internal reset as
V
voltage (V
= 120-128 µs for the LM8500 and t
LM8300. Once the t
released and the controller resume normal operation. This
internal reset will perform the same functions as external
reset. Once V
instruction execution begins. If, however, V
the selected V
to 120-128 µs for the LM8500 and 360-384 µs for the
LM8300. The device now waits until V
at which time the countdown starts over. When enabled, the
functional operation of the device, at frequency, is guaran-
teed down to the V
One exception to the above is that the brownout circuit will
insert a delay of approximately 3 ms on power up or any time
the V
be held in Reset for the duration of this delay before t
count down. This delay starts as soon as the V
above the trigger voltage (approximately 1.8V). This behav-
ior is shown in Figure 4.
In Case 1, V
undefined until the supply is greater than approximately
1.0V. At this time the brownout circuit becomes active and
holds the device in RESET. As the supply passes a level of
about 1.8V, a delay of about 3 ms (td) is started and t
preset with 120-128 µs for the LM8500 or 360-384 µs for the
LM8300. Once V
t
Case 2 shows a subsequent dip in the supply voltage which
goes below the approximate 1.8V level. As V
V
back above the 1.8V level, td is started. Since the power
supply rise time is longer for this case, td has expired before
V
greater than V
Case 3 shows a dip in the supply where V
V
V
goes back above V
id
CC
bor
CC
bor
CC
starts to count down.
, the internal RESET signal is asserted. When V
, but not below 1.8V. On-chip RESET is asserted when
rises above V
goes below V
rises. While V
CC
FIGURE 3. Reset Circuit using External Reset
drops below a voltage of about 1.8V. The device will
bor
), the device is held in the reset condition for t
CC
bor
bor
CC
, an internal reset is generated, and t
CC
.
rises from 0V and the on-chip RESET is
is above the V
bor
bor
bor
bor
CC
is greater than V
id
and t
.
level.
and t
is less than the specified brownout
reaches zero, the internal reset is
id
id
starts immediately when V
starts as soon as the supply
bor
id
bor
CC
and t
= 360-384 µs for the
is greater than V
and td has expired,
id
CC
CC
CC
reaches zero,
drops below
drops below
drops below
20037205
CC
CC
id
id
starts
is set
CC
rises
rises
id
bor
is
is
id
,