TC818A datasheet(13/24 Pages) MICROCHIP

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DS21475B-page 13

TC818A

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

EXAMPLE 3-2:

IN FIXED RANGE

(200 FULL SCALE)

EXAMPLE 3-3:

IN FIXED RANGE 20V

FULL SCALE

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.

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 10M

Ω

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 200

Ω 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 2k

Ω.

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

2000k

Ω 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.

RI (N)

= 18.21k

Ω (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

RI (N)

= 18.21k

Ω = ≥ 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.

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

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