ADP315PC87570 National Semiconductor, ADP315PC87570 Datasheet - Page 125

• Manufacturer Part Number: ADP315PC87570
• Description: Keyboard and Power Management Controller
• Manufacturer: National Semiconductor

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16.4.5 ADC Data Registers
The ADC stores the conversion results in four byte-wide,
read only registers, ADDATA0 - ADDATA3. If there is more
than one result, the results are stored in ADDATA0,
ADDATA1, ADDATA2 and ADDATA3 (in this order).
Data is valid only after the EOC flag of ADCST is set. Upon
cold reset (power-up), the contents of these registers is un-
defined. Upon warm reset (using HMR pin), the content of
these registers is not modified. The registers should be ini-
tialized to 00h before enabling them.
16.5 USAGE HINTS
16.5.1 Power Supply and Layout Guidelines
The ADC and the other analog modules are supplied
through two dedicated analog power pins: AV
AGND. This assures effective isolation of the analog mod-
ules from noise caused by the digital modules. To obtain the
best performance, bear in mind the following recommenda-
tions (see also details in Figure 16-4):
Ground Connection. The analog ground pin, AGND,
should be connected at only one point to the digital ground
pins. At this point, also connect the decoupling capacitor of
the analog supply AV
erence voltage V
tors of the digital supply V
the input signals to the ADC should be the same common
point. Low impedance ground layers will also improve noise
isolation.
Power Connection. The analog supply pin, AV
be connected to a low noise power supply with the same
voltage as the digital supply, either 3.3V or 5.0V. Both the
digital and analog power supplies must be supplied simulta-
neously. To assure this, it is recommended to supply the
AV
LC or RC filter. An example of an LC filter [L
is shown in Figure 16-4.
MSB
MSB
MSB
MSB
CC
7
7
7
7
pin from the digital V
6
6
6
6
REF
5
5
5
5
RESULT 0 DATA
RESULT 1 DATA
RESULT 2 DATA
RESULT 3 DATA
CC
pin, and the four decoupling capaci-
pin, decoupling capacitor of the ref-
4
4
4
4
CC
CC
pins. The ground reference of
of the chip, using an external
3
3
3
3
Analog to Digital Converter (ADC) - January 1998
2
2
2
2
1
and (C
1
1
1
1
CC
, should
CC
2
LSB
LSB
LSB
LSB
+C
0
0
0
0
and
3
)]
125
Decoupling Capacitors. The following decoupling capaci-
tors should be used:
Back-Drive Protection. To maintain the high performance
of the analog circuits, the PD0-7/AD0-7 and V
not back-drive protected. Therefore, the voltage on these
pins must be within the actual range of AGND and AV
it is higher, the chip may be damaged.
External circuits should not drive currents into these pins
when the PC87570 is not powered up. This may cause the
internal power-up reset circuit to fail.
16.5.2 Power Consumption
ADC power consumption from AV
ADC is disabled by clearing the ADCEN bit of the
ADCCNT1 Register. The internal reference is automatically
disabled when ADCEN=0. See Section 16.2.7 on page 121.
When the ADC is enabled, the current consumption de-
pends on the operation mode and the frequency at which it
works. Typical current consumption of the ADC while it is
enabled, but not converting, is 1 mA at AV
converting in continuous mode, the current consumption is
variable, although typically lower than 2 mA. Typical current
consumption of the internal reference, when enabled, is
0.2 mA.
To minimize current consumption, disable the ADC when
not in use. See details in Section 16.2.7.
Figure 16-4. ADC Analog Power Supply Connection
Analog
Power
V
Digital V
V
place one 10-47 F tantalum capacitor (C
common net, as close as possible to the chip.
Analog AV
47 F tantalum capacitor on the AV
as close as possible to the pin.
V
pacitor (C
IN
CC
REF
Analog Ground Layer
C
C
1
2
pin, as close as possible to the pin, (4 x C
: Place a low leakage, non-polarized, 0.47 F ca-
22
0.47
+
F
F
V
C
AD0
AD7
CC
L
REF
3
1
1
) as close as possible to the V
0.1
CC
: Place one capacitor of 0.1 F on each
10-100 H
F
: Place a 0.1 F capacitor and a 10-
AD0
AD7
V
AV
AGND
REF
CC
PC87570
CC
Digital Ground Layer
GND
V
is practically zero if the
CC
CC
pin (C
C
CC
4
REF
REF
=5V. While
0.1
F
5
3
Digital
Power
) on the
pin.
+
pins are
and C
4
). Also
3.3 V
5.0 V
CC
C
22
or
5
F
. If
2
)