ADPD1081 datasheet(53/74 Pages) AD

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Data Sheet

ADPD1080/ADPD1081

Rev. C | Page 53 of 74

Monitoring Ambient Light Levels in Float LED Mode

In real-world applications, it is common for the ambient light

levels to change constantly. When using float LED mode,

increasing the amount of ambient light can approach levels

where the ambient light uses an unacceptable amount of

dynamic range of the charge that can be stored on the photodiode

capacitance. For this reason, it is required that the ambient light

level is monitored so that configuration changes can be made

when necessary, for example, float time, TIA gain, and

operating mode. There are two ways to monitor ambient light

levels. One way is to use TIA ADC mode in the alternate time

slot and continuously monitor the ambient light level. The

other way is to use a feature of the ADPD1080/ADPD1081

where the ambient light level is automatically monitored in the

background during float mode operation and is compared

against a user-defined threshold. If the ambient light level

exceeds this threshold by some user-defined number of times,

the device sets a flag that can be read by the user or can be

output to a GPIO. Table 35 lists all the registers used to

monitor the ambient light level while in float LED mode.

The user sets an ambient level threshold in the BG_THRESH

subtract cycles in float LED mode are compared against. The

subtract cycles in float LED mode are the positions in the pulse

sequence in which the LED pulse is masked; therefore, it is the

background level measurement. The ADC result is equal to the

raw ADC output minus the contents of the ADC offset register

(Register 0x18 to Register 0x1B and Register 0x1E to Register 0x21).

In the BG_COUNT register, the user sets a limit on the number

of cycles that BG_THRESH is exceeded by the ADC result before

the BG_STATUS bit is set for any particular channel. Every time

the BG_THRESH value is exceeded by the ADC result during a

subtract cycle, an internal counter increments. Each channel

has its own counter. When this count exceeds the limit set in

the BG_COUNT register, the BG_STATUS bit is set for the

channel. The user can periodically monitor the BG_STATUS

be asserted if a BG_STATUS flag is set. See Table 35 for

the various logical combinations of BG_STATUS flags and

interrupts that can be brought out on a GPIOx.

Table 35. Registers for Monitoring the Ambient Light Level in Float LED Mode

Float Mode Register Name

Time Slot A

Time Slot B

Description

BG_STATUS_x

0x04, Bits[3:0]

0x04, Bits[7:4]

Status of comparison between background light level and background

threshold value (BG_THRESH). A 1 in any bit location means the threshold

has been crossed BG_COUNT number of times. This register is cleared once

it is read.

Bit 0: Time Slot A, Channel 1 exceeded threshold count.

Bit 1: Time Slot A, Channel 2 exceeded threshold count.

Bit 2: Time Slot A, Channel 3 exceeded threshold count.

Bit 3: Time Slot A, Channel 4 exceeded threshold count.

Bit 4: Time Slot B, Channel 1 exceeded threshold count.

Bit 5: Time Slot B, Channel 2 exceeded threshold count.

Bit 6: Time Slot B, Channel 3 exceeded threshold count.

Bit 7: Time Slot B, Channel 4 exceeded threshold count.

BG_THRESH_x

0x16, Bits[13:0]

0x1C[13:0]

The background threshold that is compared against the ADC result during

the subtract cycles during float mode. If the ADC result exceeds the value

in this register, BG_COUNT is incremented.

BG_COUNT_x

0x16, Bits[15:14]

0x1C[15:14]

This is the number of times the ADC value exceeds the BG_THRESH value

during the float mode subtract cycles before the BG_STATUS bit is set.

0x0: never set BG_STATUS.

0x1: set when BG_THRESH is exceeded 1 time.

0x02: set when BG_THRESH is exceeded 4 times.

0x03: set when BG_THRESH is exceeded 16 times.

GPIO0_ALT_CFG

0x0B[4:0]

GPIO0 asserts for the following conditions:

0x10: logical OR of BG_STATUS, Bits[3:0].

0x1A: logical OR of BG_STATUS, Bits[7:4].

0x1B: logical OR of BG_STATUS, Bits[7:0].

0x1C: logical OR of BG_STATUS, Bits[7:0] and INT.

GPIO1_ALT_CFG

0x0B[12:8]

GPIO1 asserts for the following conditions:

0x10: logical OR of BG_STATUS, Bits[3:0].

0x1A: logical OR of BG_STATUS, Bits[7:4].

0x1B: logical OR of BG_STATUS, Bits[7:0].

0x1C: logical OR of BG_STATUS, Bits[7:0] and INT.

ADPD1081 Datasheet(PDF) 53 Page - Analog Devices

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