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Auto Channel Select Mode

Start of a Conversion

Status Output EOC/INT

ADS7229

ADS7230

www.ti.com........................................................................................................................................................................................................ SBAS437 – MAY 2008

Channel selection can also be done automatically if auto channel select mode is enabled. This mode is the

default channel select mode. The dual channel converter, ADS7230, has an onboard 2-to-1 MUX. If the device is

programmed for auto channel select mode, then signals from channel 0 and channel 1 are acquired with a fixed

order. Channel 0 is accessed first in the next cycle after the command cycle that configured CFR_D11 to '1' for

auto channel select mode. This automatic access stops the cycle after the command cycle that sets CFR_D11 to

'0'.

The end of sampling instance (EOS) or acquisition is the same as the start of a conversion. This event 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 a simultaneous sample/hold with multiple converters. The ADS7229/30

switches from sample to hold mode on the falling edge of the CONVST signal. The ADS7229/30 requires 18

conversion clock (CCLK) edges to complete a conversion. The conversion time is equivalent to 857ns with a

21MHz internal clock. The minimum time between two consecutive CONVST signals is 21 CCLKs.

A conversion can also be initiated without using CONVST if it is so programmed (CFR_D9 = 0). When the

converter is configured as an auto trigger, the next conversion automatically starts three conversion clocks

(CCLK) after the end of a conversion. These three conversion clocks are used as 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

Automatic No need to write channel number to the CMR. Use internal sequencer for the

Start a conversion based on the conversion

ADS7230.

clock CCLK.

Manual Channel Select

Manual Trigger

Manual

Write the channel number to the CMR.

Start a conversion with CONVST.

(1)

Auto channel select should be used with the TAG bit enabled.

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 after CONVST goes low when the manual trigger is

programmed. EOC stays low throughout the conversion process and returns high when the conversion ends.

The EOC output goes low for three conversion clocks after the previous rising edge of EOC, if auto trigger is

programmed.

This status pin is programmable. It can be used as an EOC output (CFR_D[7:6] = 1, 1) where the low time is

equal to the conversion time. This status pin can also be used as INT (CFR_D[7:6] = 1, 0), which is set low as

the end of a conversion is brought high (cleared) by the next read cycle. The polarity of this pin, used as either

function (EOC or INT), is programmable through CFR_D7.

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