HC705JJ7GRSSLASHD datasheet(120/230 Pages) NXP

120 / 230 page

Analog Subsystem

General Release Specification

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

Analog Subsystem

The above 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.

The full scale voltage range for a given capacitance, fOSC, conversion

method, and resolution is defined as:

VFS = NFS x P x ICHG / (CEXT x fOSC)

Once charged to a given voltage a finite amount of time will be required

to discharge the capacitor back to its start voltage at VSS. 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 VSS 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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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