HCPL-7860 Datasheet, PDF(20/28 Page) Agilent(Hewlett-Packard)

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HCPL-7860 Datasheet, PDF (20/28 Pages) Agilent(Hewlett-Packard) – Isolated 15-bit A/D Converter

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Digital Interface

Timing

Power Up/Reset

At power up, the digital interface

IC should be reset either

manually, by bringing the RESET

pin (pin 9) high for at least

100 ns, or automatically by

connecting a 10 ÂµF capacitor

between the RESET pin and VDD

(pin 16). The RESET pin operates

asynchronously and places the IC

in its default configuration, as

specified in the Digital Interface

Configuration section.

Conversion Timing

Figure 19 illustrates the timing

for one complete conversion

cycle. A conversion cycle is

initiated on the falling edge of the

convert start signal (CS); CS

should be held low during the

entire conversion cycle. When CS

is brought low, the serial output

data line (SDAT) changes from a

high-impedance to the low state,

indicating that the converter is

busy. A rising edge on SDAT

indicates that data is ready to be

clocked out. The output data is

clocked out on the negative edges

of the serial clock pulses (SCLK),

MSB first. A total of 16 pulses is

needed to clock out all of the data.

After the last clock pulse, CS

should be brought high again,

causing SDAT to return to a high-

impedance state, completing the

conversion cycle. If the external

circuit uses the positive edges of

SCLK to clock in the data, then a

total of sixteen bits is clocked in,

the first bit is always high

(indicating that data is ready)

followed by 15 data bits. If fewer

than 16 cycles of SCLK are input

before CS is brought high, the

conversion cycle will terminate

and SDAT will go to the high-

impedance state after a few

cycles of the Isolated Modulatorâs

clock.

The amount of time between the

falling edge of CS and the rising

edge of SDAT depends on which

conversion and pre-trigger modes

are selected; it can be as low as

0.7 Âµs when using pre-trigger

mode 2, as explained in the

Digital Interface Configuration

section.

Serial Configuration

Timing

The HCPL-x870 Digital Interface

IC is programmed using the

Serial Configuration Interface

(SCI) which consists of the clock

(CCLK), data (CDAT), and

enable/latch (CLAT) signals.

Figure 18 illustrates the timing

for the serial configuration inter-

face. To send a byte of configura-

tion data to the HCPL-x870, first

bring CLAT low. Then clock in

the eight bits of the configuration

byte (MSB first) using CDAT and

the rising edge of CCLK. After the

last bit has been clocked in,

bringing CLAT high again will

latch the data into the appropri-

ate configuration register inside

the interface IC. If more than

eight bits are clocked in before

CLAT is brought high, only the

last eight bits will be used. Refer

to the Digital Interface Configura-

tion section to determine appro-

priate configuration data. If the

default configuration of the

digital interface IC is acceptable,

then CCLK, CDIN and CLAT may

be connected to either VDD or

GND.

Channel Select Timing

The channel select signal (CHAN)

determines which input channel

will be used for the next conver-

sion cycle. A logic low level

selects channel one, a high level

selects channel 2. CHAN should

not be changed during a conver-

sion cycle. The state of the CHAN

signal has no effect on the

behavior of either the over-range

detection circuit (OVR1) or the

adjustable threshold detection

circuit (THR1). Both OVR1 and

THR1 continuously monitor

channel 1 independent of the

CHAN signal. CHAN also does not

affect the behavior of the pre-

trigger circuit, which is tied to

the conversion timing of channel

1, as explained in the Digital

Interface Configuration section.

Digital Interface

Configuration

Configuration Registers

The Digital Interface IC contains

four 6-bit configuration registers

that control its behavior. The two

LSBs of any byte clocked into the

serial configuration port (CDAT,

CCLK, CLAT) are used as address

bits to determine which register

the data will be loaded into.

Registers 0 and 1 (with address

bits 00 and 01) specify the

conversion and offset calibration

modes of channels 1 and 2,

specifies the behavior of the

adjustable threshold circuit, and

specifies which pre-trigger mode

to use for channel 1. These

registers are illustrated in Table 3

below, with default values

indicated in bold italic type. Note

that there are several reserved

bits which should always be set

low and that the configuration

registers should not be changed

during a conversion cycle.

1-279

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