TC818ACBU View Datasheet(PDF) - Microchip Technology
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TC818ACBU Datasheet PDF : 24 Pages
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3.11 -MEM Operating Mode
Bringing -MEM (Pin 61) momentarily LOW configures
the TC818A “-MEM” Operating mode. The -MEM LCD
annunciator becomes active. In this Operating mode,
subsequent measurements are made relative to the
last two digits (≤99) displayed at the time MEM is LOW.
This represents 5% of full scale. The last two significant
digits are stored and subtracted from all the following
input conversions.
The following examples clarify operation:
EXAMPLE 3-1: IN AUTO-RANGING
RI (N) = 18.21kΩ (20kΩ Range) = ≥ Display 18.21kΩ
MEM = ≥ Store 0.21kΩ
RI (N+1)= 19.87kΩ (20kΩ Range)
= ≥ Display 19.87 - 0.21 = 19.66kΩ
RI (N+2)= 22.65kΩ (200kΩ Range)
= ≥ Display 22.7kΩ and MEM disappears
EXAMPLE 3-2:
IN FIXED RANGE
(200 FULL SCALE)
RI (N) = 18.21kΩ = ≥ Display 18.2kΩ
MEM = ≥ Store 8.2Ω
RI (N+1) = 36.7Ω
= ≥ Display 36.7 - 8.2 = 28.5Ω
RI (N+2) = 5.8Ω
= ≥ Display 5.8-8.2 = -2.4Ω*
*Will display minus resistance if following input is less than
offset stored at fixed range.
EXAMPLE 3-3:
IN FIXED RANGE 20V
FULL SCALE
VI (N) = 0.51V = ≥ Display 0.51V
MEM = ≥ Store 0.51V
VI (N+1) = 3.68V
= ≥ Display 3.68 - 0.51 = 3.17V
VI (N+2) = 0.23V
= ≥ Display 0.23 -0.51 = -0.28V
VI (N+3) = -5.21V
= ≥ Display - 5.21 - 0.51 = -5.72V
On power-up, the “-MEM” mode is not active. Once the
“-MEM” is entered, bringing MEM LOW again returns
the TC818A to normal operation.
The “-MEM” mode is also cancelled whenever the mea-
surement type (resistance, voltage, current AC/DC,
Ω/LOΩ) or range is changed. The LCD -MEM annunci-
ator will be off in normal operation.
In the auto-range operation, if the following input signal
cannot be converted on the same range as the stored
value, the “-MEM” mode is cancelled. The LCD annun-
ciator is turned OFF.
TC818A
The “-MEM” Operating mode can be very useful in
resistance measurements when lead length resistance
would cause measurement errors.
3.12 Automatic Range Selection
Operation
When power is first applied, the TC818A enters the
auto-range operating state. The Auto-Range mode
may be entered from Manual mode by changing the
measurement function (resistance or voltage), or by
changing the measurement option (AC/DC, Ω/LOΩ).
The automatic voltage range selection begins on the
most sensitive scale first: 200mV for DC or 2V for AC
measurements. The voltage range selection flow chart
is shown in Figure 3-8.
Internal input protection diodes to VCC (Pin 28) and VSS
(Pin 57) clamp the input voltage. The external 10MΩ
input resistance (see R14 and R13 of Functional dia-
gram) limits current safely in an over range condition.
The voltage range selection is designed to maximize
resolution. For input signals less than 9% of full scale
(count reading <180), the next most sensitive range is
selected.
An over range voltage input condition is flagged when-
ever the internal count exceeds 2000, by activating the
buzzer output (Pin 4). This 4kHz signal can directly
drive a piezo electric acoustic transducer. An out of
range input signal causes the 4kHz signal to be on for
122msec, off for 122msec, on for 122msec and off for
610msec (see Figure 3-14).
During voltage auto-range operation, the extended res-
olution feature operates on the 2000V range only (see
Extended Resolution Operating mode discussion).
The resistance automatic range selection procedure is
shown in Figure 3-9. The 200Ω range is the first range
selected, unless the LOW ohms resistance measure-
ment option is selected. In LOW ohms operation, the
first full scale range tried is 2kΩ.
The resistance range selected maximizes sensitivity. If
the conversion results in a reading less than 180, the
next most sensitive full scale range is tried.
If the conversion is less than 19 in auto-range opera-
tion, a continuous 4kHz signal is output at BUZ (Pin 4).
An over range input does not activate the buzzer.
Out-of-range input conditions are displayed by a blink-
ing MSD with the three LSDs set to “000.”
The extended resolution feature operates only on the
2000kΩ and 2000V full scale range during auto-range
operation. A blinking “1” most significant digit is inter-
preted as the digit 2. The three least significant digits
display data normally.
© 2002 Microchip Technology Inc.
DS21475B-page 13
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