参数资料
型号: MC68HC705JJ7S
厂商: FREESCALE SEMICONDUCTOR INC
元件分类: 微控制器/微处理器
英文描述: 8-BIT, UVPROM, 2.1 MHz, MICROCONTROLLER, CDIP20
封装: WINDOWED, CERAMIC, DIP-20
文件页数: 146/164页
文件大小: 1165K
代理商: MC68HC705JJ7S
Analog Subsystem
MC68HC705JJ7 MC68HC705JP7 Advance Information Data Sheet, Rev. 4.1
82
Freescale Semiconductor
these bits and the ISEN bit are cleared when the device is reset, the MC68HC705JJ7/MC68HC705JP7
starts with the charge and discharge function disabled.
The length of time required to reach the maximum voltage to be measured and the speed of the time
counting mechanism will determine the resolution of the reading. The time to ramp the external capacitor
voltage to match the maximum voltage is dependent on:
Charging current to external capacitor
Value of the external capacitor
Clock rate for timing function
Any prescaling of the clock to the timing function
Desired resolution
The charging behavior is described by the general equation:
tCHG = CEXT x VX / ICHG
Where:
tCHG= Charge time (seconds)
CEXT= Capacitance (F)
VX= Unknown voltage (volts)
ICHG= Charge current (A)
Since the MCU can measure time in a variety of ways, the resolution of the conversion will depend on the
length of the time keeping function and its prescaling to the oscillator frequency (fOSC). Therefore, the
charge time also equals:
tCHG = P x N / fOSC
Where:
P= Prescaler value (
÷ 2, ÷ 4, ÷ 8, etc.)
N= Number of counts during charge time
fOSC= Oscillator clock frequency (Hz)
NOTE
Noise on the system ground or the external ramping capacitor can cause
the comparator to trip prematurely. Therefore, in any given application it is
best to use the fastest possible ramp rate (shortest charge time).
The previous two equations for the charge time, tCHG, can be combined to form the following expression
for the full scale count (NFS) of the measured time versus the full scale unknown voltage (VFS):
NFS = CEXT x VFS x fOSC / (P x ICHG)
Since a given timing method has a fixed charge current and prescaler, then the variation in the resultant
count for a given unknown voltage is mainly dependent on the operating frequency and the capacitance
value used. The desired external capacitance for a given voltage range, fOSC, conversion method, and
resolution is defined as:
CEXT = NFS x P x ICHG / (VFS x fOSC)
NOTE
The value of any capacitor connected directly to the PB0/AN0 pin should be
limited to less than 2 microfarads. Larger capacitances will create high
discharge currents which may damage the device or create signal noise.
相关PDF资料
PDF描述
MC68HC705K1DWR2 8-BIT, OTPROM, MICROCONTROLLER, PDSO16
MC68HC705K1CP 8-BIT, OTPROM, MICROCONTROLLER, PDIP16
MC68HC705K1P 8-BIT, OTPROM, 2 MHz, MICROCONTROLLER, PDIP16
MC68HC705K1DW 8-BIT, OTPROM, MICROCONTROLLER, PDSO16
MC68HC705KJ1CDW 8-BIT, OTPROM, 4 MHz, MICROCONTROLLER, PDSO16
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