Datasheet

Alarm

ADC Output

H_ALARM On

H_ALARM Off

(T – H)

(T + H + 1)

ADC Output

L_ALARM Off

L_ALARM On

(T + H)(T – H – 1)

Alarm

ADS8634

ADS8638

SBAS541A –MAY 2011– REVISED AUGUST 2011

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AL_PD: USER-CONFIGURABLE PIN

The ADS8634/8 feature a user-configurable AL_PD pin. This pin can either be configured as an alarm output

(AL) or as a power-down control pin (PD). Refer to the Page 0, Register Descriptions for the ADS8638 and Page

0, Register Descriptions for the ADS8634 sections for details.

When programmed as an alarm output, an active-high alarm is flagged on this pin if there is a high or low alarm

on any channel. The Alarm Functionality section describes the pin details.

When programmed as PD, the AL_PD pin functions as an active-low power-down input pin. Powering down

through this pin is asynchronous. The devices power down immediately after the pin goes low. The Power-Down

Functionality section describes the pin details.

This pin is configured as a PD input by default after power-on or reset.

Alarm Functionality

The ADS8634/8 output an active-high alarm on the AL_PD pin when it is programmed as an AL. AL is

synchronous and changes its state on the 16th SCLK rising edge. A high level on AL indicates there is an active

alarm on one or more channels. This pin can be wired to interrupt the host input. When an alarm interrupt is

received, the alarm flag registers are read to determine which channels have an alarm.

The ADS8634/8 feature independently-programmable alarms for each channel. There are two alarms per

channel (low and high alarm) and each alarm threshold has a separate hysteresis setting.

The ADS8634/8 set a high alarm when the digital output for a particular channel exceeds the high alarm upper

limit (high alarm threshold T + hysteresis H). The alarm resets when the digital output for the channel is less than

or equal to the high alarm lower limit (high alarm T – H). This function is shown in Figure 78.

NOTE: T = alarm threshold and H = hysteresis.

Figure 78. High-Alarm Hysteresis

Similarly, the lower alarm is triggered when the digital output for a particular channel falls below the low alarm

lower limit (low alarm threshold T – H). The alarm resets when the digital output for the channel is greater than or

equal to the low alarm higher limit (low alarm T + H). This function is shown in Figure 79.

NOTE: T = alarm threshold and H = hysteresis.

Figure 79. Low-Alarm Hysteresis

Product Folder Link(s): ADS8634 ADS8638