AT89LP6440 Atmel Corporation, AT89LP6440 Datasheet - Page 135

• Manufacturer Part Number: AT89LP6440
• Manufacturer: Atmel Corporation

Specifications of AT89LP6440

• Flash (kbytes): 64 Kbytes
• Max. Operating Frequency: 20 MHz
• Cpu: 8051-1C
• Max I/o Pins: 38
• Spi: 1
• Twi (i2c): 1
• Uart: 1
• Adc Channels: 8
• Adc Resolution (bits): 10
• Adc Speed (ksps): 153.8
• Sram (kbytes): 4.25
• Eeprom (bytes): 8192
• Self Program Memory: IAP
• Operating Voltage (vcc): 2.4 to 3.6
• Timers: 3
• Isp: SPI/OCD
• Watchdog: Yes

20.1
3706C–MICRO–2/11
ADC Operation
The ADC converts an analog input voltage to a 10-bit signed digital value through successive
approximation. When DIFF (DADI.3) is zero, the ADC operates in single-ended mode and the
input voltage is the difference between the voltage at the input pin and V
mode (DIFF = 1) the input voltage is the difference between the positive and negative input pins.
The minimum value represents zero difference and the maximum values represent a difference
of positive or negative V
The analog input channel is selected by writing to the ACS bits in DADI. Any of the eight Port 0
input pins can be selected as single-ended inputs to the ADC. Four pairs of Port 0 pins can be
selected as differential inputs. The ACON bit (DADI.7) must be set to one to connect the input
pins to the ADC. Prior to changing ACS, ACON must be cleared to zero. This ensures that
crosstalk between channels is limited. ACON must be set back to one after ACS is updated.
ACON and ACS should not be changed while a conversion is in progress. ADC input channels
must have their port pins configured for input-only mode.
The ADC is enabled by setting the ADCE bit in DADC. Some settling time is required for the ref-
erence circuits to stabilize after the ADC is enabled. The ADC does not consume power when
ADCE is cleared, so it is recommended to switch off the ADC before entering power saving
modes.
A timing diagram of an ADC conversion is shown in
ADC clock cycles to complete. The analog input is sampled during the third cycle of the conver-
sion and is held constant for the remainder of the conversion. At the end of the conversion, the
interrupt flag, ADIF, is set and the result is written to the data registers. An additional 1 ADC
clock cycle and up to 2 system clock cycles may be required to synchronize ADIF with the rest of
the system. The results in DADH/DADL remain valid until the next conversion completes. DADH
and DADL are read-only registers during ADC mode.
Figure 20-2. ADC Timing Diagram
The equivalent model for the analog input circuitry is illustrated in
applied to ADCn is subjected to the pin capacitance and input leakage of that pin, regardless of
whether that channel is selected as input to the ADC. When the channel is selected, the source
must drive the S/H capacitor through the series resistance (combined resistance in the input
path). To achieve 10-bit resolution the S/H capacitor must be charged to within 1/2 LSB of the
expected value within the 1 ADC clock period sample time. High impedance sources may
require a reduction in the ADC clock frequency to achieve full resolution.
Cycle Number
ADC Clock
GO/BSY
ADIF
DADH
DADL
1
2
Initialize Circuitry
REF
3
minus 1 LSB.
4
5
Sample & Hold
One Conversion
6
7
8
Figure
9
10
Conversion
Complete
20-2. The conversion requires 13
AT89LP3240/6440
11
Figure
12
13
20-3. An analog source
DD
MSB of Result
LSB of Result
/2. In differential
Next Conversion
1
2
Initialize
3
135