MC68HC705JP7CDW Freescale Semiconductor, MC68HC705JP7CDW Datasheet - Page 90

• Manufacturer Part Number: MC68HC705JP7CDW
• Description: IC MCU 8BIT 28-SOIC
• Manufacturer: Freescale Semiconductor
• Series: HC05r

Specifications of MC68HC705JP7CDW

• Core Processor: HC05
• Core Size: 8-Bit
• Speed: 2.1MHz
• Connectivity: SIO
• Peripherals: POR, Temp Sensor, WDT
• Number Of I /o: 22
• Program Memory Size: 6KB (6K x 8)
• Program Memory Type: OTP
• Ram Size: 224 x 8
• Voltage - Supply (vcc/vdd): 2.7 V ~ 5.5 V
• Data Converters: A/D 4x12b
• Oscillator Type: Internal
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 28-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Eeprom Size: -

Analog Subsystem
8.6.1 Absolute Voltage Readings
The absolute value of a voltage measurement can be calculated in software by first taking a reference
reading from a fixed source and then comparing subsequent unknown voltages to that reading as a
percentage of the reference voltage multiplied times the known reference value.
The accuracy of absolute readings will depend on the error sources taken into account using the features
of the analog subsystem and appropriate software as described in
table, most of the errors can be reduced by frequent comparisons to a known voltage, use of the inverted
comparator inputs, and averaging of multiple samples.
8.6.1.1 Internal Absolute Reference
If a stable source of V
case, the reference reading can be taken by setting the V
AMUX register. This connects the channel selection bus to the V
the DHOLD bit should be used to select the 1/2 divided input.
8.6.1.2 External Absolute Reference
If a stable external source is provided, the reference measurement point can be any one of the channel
selected pins from PB1–PB4. In this case the reference reading can be taken by setting the MUX bit in
the AMUX which connects channel selection bus to the pin connected to the external reference source.
If the external reference is greater than V
1/2 divided input.
90
Change in reference voltage Provide closer tolerance reference
Change in magnitude of
ramp current source
Non-linearity of ramp
current source vs. voltage
Frequency shift in internal
low-power oscillator
Sampling capacitor leakage
Internal voltage divider ratio
Input offset voltage of
comparator 2
Noise internal to MCU
Error Source
MC68HC705JJ7 • MC68HC705JP7 Advance Information Data Sheet, Rev. 4.1
DD
is provided, the reference measurement point can be internally selected. In this
Not adjustable
Not adjustable
Use external oscillator with crystal
Use faster conversion times
Not adjustable
Not adjustable
Close decoupling at V
reduce supply source impedance
Table 8-6. Absolute Voltage Reading Errors
In Hardware
DD
DD
–1.5 volts, then the DHOLD bit should be used to select the
and V
Accuracy Improvements Possible
SS
pins and
REF
bit and clearing the MUX1:4 bits in the
Calibration and storage of reference source over
temperature and supply voltage
Compare unknown with recent measurement
from reference
Calibration and storage of voltages at 1/4, 1/2,
3/4, and FS
Compare unknown with recent measurement
from reference
Compare unknown with recent measurement
from reference
Compare unknown with recent measurement
from reference OR avoid use of divided input
Sum two readings on reference or unknown
using INV and INV control bit and divide by 2
(average of both)
Average multiple readings on both the reference
and the unknown voltage
DD
Table
pin. To stay within the V
8-6. As can be seen from this
In Software
Freescale Semiconductor
MAX
range,