MC68HC705JP7CDW Freescale Semiconductor, MC68HC705JP7CDW Datasheet - Page 91

• 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: -

8.6.2 Ratiometric Voltage Readings
The ratiometric value of a voltage measurement can be calculated in software by first taking a reference
reading from a reference source and then comparing subsequent unknown voltages to that reading as a
percentage of the reference value. The accuracy of ratiometric readings will depend on the variety of
sources, but will generally be better than for absolute readings. Many of these error sources can be
taken into account using the features of the analog subsystem and appropriate software as described in
Table
to the reference voltage, use of the inverted comparator inputs, and averaging of multiple samples.
8.6.2.1 Internal Ratiometric Reference
If readings are to be ratiometric to V
this case the reference reading can be taken by setting the V
AMUX register which connects the channel selection bus to the V
range, the DHOLD bit should be used to select the 1/2 divided input.
8.6.2.2 External Ratiometric Reference
If readings are to be ratiometric to some external source, the reference measurement point can be
connected to 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
bit should be used to select the 1/2 divided input.
Freescale Semiconductor
Change in reference voltage
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
8-7. As with absolute measurements, most of the errors can be reduced by frequent comparisons
Error Source
MC68HC705JJ7 • MC68HC705JP7 Advance Information Data Sheet, Rev. 4.1
Table 8-7. Ratiometric Voltage Reading Errors
Not required for ratiometric
Not adjustable
Not adjustable
Not required for ratiometric
Use faster conversion times
Not adjustable
Not adjustable
Close decoupling at V
reduce supply source impedance
DD
, the reference measurement point can be internally selected. In
In Hardware
DD
and V
Accuracy Improvements Possible
SS
pins and
REF
DD
bit and clearing the MUX1:4 bits in the
Compare unknown with recent measurement
from reference
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
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
pin. In order to stay within the V
DD
–1.5 volts, then the DHOLD
Voltage Measurement Methods
In Software
MAX
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