Datasheet

ADS8331

ADS8332

SBAS363C –DECEMBER 2009–REVISED MAY 2012

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Start of a Conversion

The end of acquisition is the same as the start of a conversion. This process is initiated by bringing the CONVST

pin low for a minimum of 40ns. After the minimum requirement has been met, the CONVST pin can be brought

high. CONVST acts independently of FS/CS so it is possible to use one common CONVST for applications that

require simultaneous sample/hold with multiple converters. The ADS8331/32 switches from sample to hold mode

on the falling edge of the CONVST signal. The ADS8331/32 requires 18 conversion clock (CCLK) cycles to

complete a conversion. The conversion time is equivalent to 1500ns with a 12MHz internal clock. The minimum

time between two consecutive CONVST signals is 21 CCLKs.

A conversion can also be initiated without using CONVST if the ADS8331/32 is programmed for Auto-Trigger

mode (CFR_D9 = '0'). When the converter is configured in this mode, and with CFR_D8 = '0', the next

conversion is automatically started three conversion clocks (CCLK) after the end of a conversion. These three

conversion clocks (CCLK) are used for the acquisition time. In this case, the time to complete one acquisition

and conversion cycle is 21 CCLKs. Table 1 summarizes the different conversion modes.

Table 1. Different Types of Conversion

MODE SELECT CHANNEL START CONVERSION

Auto Channel Select

(1)

Auto-Trigger Mode

Automatic

No need to write channel number to CMR. Use internal sequencer for Start a conversion based on conversion

ADS8331/32. clock CCLK

Manual Channel Select Manual-Trigger Mode

Manual

Write channel number to CMR Start a conversion with CONVST

(1) Auto channel select should be used with Auto-Trigger mode and TAG bit output enabled.

Manual Channel select with Auto-Trigger mode enabled is generally used when continuous conversions from a

single channel are desired. In this mode, cycling the input mux to change the channel requires that conversions

are halted by setting the converter to Manual-Trigger mode. When the proper input channel is selected, the

converter can be placed back to Auto-Trigger mode to continue continuous conversions from the new channel.

Status Output Pin (EOC/INT)

The status output pin is programmable. It can be used as an EOC output (CFR_D[7:6] = '11') where the low time

is equal to the conversion time. When the status pin is programmed as EOC and the polarity is set as active low,

the pin works in the following manner: the EOC output goes low immediately following CONVST going low with

Manual-Trigger mode enabled. EOC stays low throughout the conversion process and returns high when the

conversion has ended. If Auto-Trigger mode is enabled, the EOC output remains high for three conversion clocks

(CCLK) after the previous rising edge of EOC .

This status pin can also be used as an interrupt output, INT (CFR_D[7:6] = '10'), which is set low at the end of a

conversion, and is brought high (cleared) by the next read cycle. The polarity of this pin, whether used as EOC

or INT, is programmable through the CFR_D7 bit.