Datasheet

Auto Channel Select Mode
Start of a Conversion
Status Output EOC/ INT
ADS7229
ADS7230
SBAS437A – MAY 2008 – REVISED JUNE 2009 ............................................................................................................................................................... www.ti.com
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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