MC68HC705JJ7 Datasheet, PDF (83/164 Pages) Freescale Semiconductor, Inc – Microcontrollers

A/D Conversion Methods
The full scale voltage range for a given capacitance, f_OSC, conversion method, and resolution is defined
as:
V_FS = N_FS x P x I_CHG / (C_EXT x f_OSC)
Once charged to a given voltage, a finite amount of time will be required to discharge the capacitor back
to its start voltage at V_SS. This discharge time will be solely based on the value of capacitance used and
the sinking current of the internal discharge device. To allow a reasonable time for the capacitor to return
to V_SS levels, the discharge time should last about 10 milliseconds per microfarad of capacitance
attached to the PB0 pin. If the total charge/discharge cycle time is critical, then the discharge time should
be at least 1/10 of the most recent charge time. Shorter discharge times may be used if lesser accuracy
in the voltage measurement is acceptable.
NOTE
Sufficient time should be allowed to discharge the external capacitor or
subsequent charge times will be shortened with resultant errors in timing
conversion.
Table 8-4 gives the range of values of each parameter in the A/D timing conversion and Table 8-5 gives
some A/D conversion examples for several bit resolutions.
The mode selection bits in the ACR allow four methods of single-slope A/D conversion. Each of these
methods is shown in Figure 8-8 through Figure 8-11 using the signal names and parameters given in
Table 8-4.
• Manual start and stop (mode 0) Figure 8-8
• Manual start and automatic discharge (mode 1) Figure 8-9
• Automatic start and stop from TOF to ICF (mode 2) Figure 8-10
• Automatic start and stop from OCF to ICF (mode 3) Figure 8-11
Table 8-4. A/D Conversion Parameters
Name
V_X
V_MAX
I_CHG
Function
Unknown voltage on channel selection bus
Maximum charging voltage on external capacitor
Charging current on external ramping capacitor
V_DD = 3 Vdc
V_DD = 5 Vdc
I_DIS Discharge current on external ramping capacitor
t_CHG
t_DIS
C_EXT
N
Time to charge external capacitor
(100 kHz < f_OSC < 4.0 MHz)
4-bit result
6-bit result
8-bit result
10-bit result
12-bit result
Time to discharge external capacitor, C_EXT
Capacitance of external ramping capacitor
Number of counts for I_CHG to charge C_EXT to V_X
Min
Typ
Max
Units
V_SS
—
V_DD –1.5
V
—
—
V_DD –1.5
V
Refer to 15.9 Analog Subsystem Characteristics (5.0 Vdc) and
15.10 Analog Subsystem Characteristics (3.0 Vdc)
Refer to 15.9 Analog Subsystem Characteristics (5.0 Vdc) and
15.10 Analog Subsystem Characteristics (3.0 Vdc)
0.032
0.128
0.512
2.048
8.192
—
0.0001
1
0.128
0.512
2.048
8.196
32.768
5
0.1
1024
2.56
10.24
ms
40.96
120⁽¹⁾
120⁽¹⁾
10
ms/µF
2.0
µF
65536
Counts
Continued on next page
MC68HC705JJ7 • MC68HC705JP7 Advance Information Data Sheet, Rev. 4.1
Freescale Semiconductor
83