FUNCTIONAL DESCRIPTION (cont.)

HOLT INTEGRATED CIRCUITS

5

WETTING CURRENT

In Ground/Open mode a current of approximately 1mA is

sourced from the SENSE pin when it is grounded and VDD is

powered at 12.0V. The wetting current serves to provide

current through switches or relay contacts to prevent dry

contacts and improve switch contact reliability.

When programmed as Supply/Open sensors, CRn is set to a

logic 0. Referring to Figure 1,

Supply/Open thresholds are set at >12V

for the supply state and <6V for the open state, providing 3 -

6V of hysteresis / noise rejection.

For the Supply/Open case the wetting current into the sense

input is simply the current sunk by the effective 30k

to

GND. For V

= 28V, I

is 1ma. See Figure 2.

SUPPLY/OPEN SENSING

WETTING CURRENT

Ω

SENSE

WET

a switch in series with a diode

is closed to provide a pull down to ground of 30k

(including

the external 3k ).

Ω

Ω

BUILT-IN TEST

Writing a high in Configuration Register bit CR9 puts the

HI-8428-R into the Built-In Test (BIT) mode. Referring to

Figure 1, when in the test mode each of the internal inputs

to the sense comparators are set to either a high or low.

Since the input sense pin is isolated by a 680k

resistor,

this test mode will not disturb the actual status of the input

pin.

When in BIT mode, setting CRn high for a particular sensor

forces the comparator inputs high. A zero in CRn forces the

comparator input low. To verify correct comparator

behavior, the user must read the Data Register and

compare with the value written to CR1-8.

Certain flight applications require periodic sensor

testing during flight.

Ω

NOTE:

To ensure seamless transition

between BIT mode and normal operation mode, the

following steps should be followed:

1) The host should read and record the Configuration

Register value for normal mode operation.

2) The host should read and record the last value of

the Data Register before enabling BIT mode

(CR9 = 1).

HI-8428-R

V

(V)

SENSE

-10

0

102030405060

1.75

1.55

1.35

1.15

0.95

0.75

0.55

0.35

0.15

-0.05

Figure 2.

Supply/Open Mode SENSE Input IV Characteristic (VDD = 15V)

V

(V)

SENSE

-10

0

102030405060

0.2

-0.3

-0.8

-1.3

-1.8

-2.3

Figure 3.

Ground/Open Mode SENSE Input IV Characteristic (VDD = 15V)

V

(V)

SENSE

-10

0

102030405060

Figure 5.

Power-Off SENSE Input IV Characteristics

350

300

250

200

150

100

50

0

-50

V

=V

=GND

DD

LOGIC

V

=V

=Open

DD

LOGIC

V

(V)

SENSE

-10

0

102030405060

60

50

40

30

20

10

0

-10

-20

Figure 4.

Hi-Z SENSE Input IV Characteristic (VDD = 15V)