ADPD4100 datasheet(20/101 Pages) AD

20 / 101 page

ADPD4100/ADPD4101

Data Sheet

Rev. 0 | Page 20 of 101

At the end of the pulse operations in each time slot, the lit and

dark values are clipped to positive numbers and are sent to the

decimation unit. At the end of time slot operations, if the

decimated value is ready, the signal value is calculated by

subtracting the dark value from the lit value. Then, the data

registers that are ready are updated, and the selected values are

written to the FIFO. The data interrupt for that time slot is also

set at this time for each updated time slot.

Decimation

The DECIMATE_FACTOR_x bits determine the number of

time slot values used to create a 32-bit final sample value at a

rate of

Sample Rate =

(1/TIMESLOT_PERIOD_x)/(DECIMATE_FACTOR_x + 1)

If DECIMATE_FACTOR_x is 0, the output sample rate equals

the time slot rate. The final value is the sum of the decimated

samples. There is no divide by (DECIMATE_FACTOR_x + 1)

operation performed on the decimated data, but final data

values can be bit shifted to the right before being written to the

FIFO, creating a direct average when the number of samples is a

power of 2. DECIMATE_TYPE_x selects the method of

decimation used. A setting of 0 selects a simple block sum with

other settings allowing higher order CIC filters up to fourth

order. If using higher order CIC filters for the signal data, the

dark data still uses the simple block sum at the same decimation

rate. Each time slot maintains its own block sum or CIC filter

state. The entire decimation path uses a 32-bit datapath. When

using the CIC filter, the number of bits required for the result is

dependent on the number of pulses, the decimation rate and the

order of the CIC filter according to the following equation:

NBITS = 14 + log2(Number of Pulses) + (log2(Decimation

Rate))(CIC Order)

It is up to the user to ensure that there is no undesired overflow.

Final data results can be read from data registers or a 512-byte

data FIFO. Data written to the FIFO is configurable to allow the

different data registers, formats, and data sizes as required. Each

time slot can use its own decimation rate. Data from each time

slot is written to the FIFO at its respective ODR.

Subsampling

The ADPD4100/ADPD4101 support a subsampling mode that

allows selected time slots to run at slower sampling rates than

the programmed sampling rate. For example, in a multiparameter

application where most of the measurements need to be taken

at a sampling rate of 300 Hz but one of the measurements only

needs to be taken at 25 Hz, the subsampling mode can be used

on the time slot that only needs to operate at 25 Hz. To enable

subsampling mode for a specific time slot, set the SUBSAMPLE_x

bit to 1 and set the DECIMATE_FACTOR_x bits to the desired

subsampling rate. The subsampled time slot then samples only

once every (DECIMATE_FACTOR_x + 1) cycles, instead of

operating every time slot sequence. If other time slots are decimat-

ing at the same rate, the subsampled cycles occur at the same

time the decimated data is presented to the FIFO. For example,

if Time Slot A is operating at 300 Hz but decimating to 25 Hz,

and Time Slot B is set to subsample by 12, both time slots write

the FIFO during the same time slot sequence and at the same rate.

More complicated patterns can be made if the decimate and

subsample rates for the enabled time slots are different. The

user must manage the varying packet sizes by reading the data

in multiples of the repeating packet size. For example, if Time

Slot A is not decimating or subsampling, Time Slot B is subsam-

pling every second cycle, and Time Slot C is subsampling every

fourth cycle, the data pattern written to the FIFO is A, AB, A,

ABC, and so on, as the repeating packet.

Decimation and subsampling have the same effect on the output

data rate. The only difference is that the decimated time slots

operate every input cycle but produce data at the slower rate

using the on-chip decimating filter. The subsampling time slots

only occur at the slower rate.

Status bytes are written to the FIFO every wake-up period,

regardless of which time slots execute. Using the same example

as the different decimate and subsample rates scenario, but with

a status byte enabled, the pattern is AS, ABS, AS, ABCS, and so

on, where S is a status byte.

FIFO

Data is written to the FIFO at the end of each sampling period.

This packet can include 0, 8-, 16-, 24-, or 32-bit data for each of

the dark data, lit data, and signal data values. The bit alignment

of the data written to the FIFO is selectable with a shift of 0 bits

to 31 bits, with saturation provided. Lower bits are ignored. The

DARK_SHIFT_x, LIT_SHIFT_x, and SIGNAL_SHIFT_x bits

select the number of bits to shift the output data to the right

before writing to the FIFO. The DARK_SIZE_x, LIT_SIZE_x,

and SIGNAL_SIZE_x bits select the number of bytes of each

field to be written from 0 bytes to 4 bytes. When set to 0, no

data is written for that data type. If there are any nonzero bits at

more significant bit positions than those selected, the data

written to the FIFO is saturated. If both channels are enabled,

all selected Channel 1 data values are written to the FIFO first,

followed by the Channel 2 data.

For example, in modes that utilize dark data, the eight upper

bits of the dark data can be stored with 24 appropriately selected

bits from the signal data for each time slot to allow detection of

whether the ambient light is becoming large, while limiting the

size of the amount of data transferred.

Data is written to the FIFO at the end of the sampling period

only if there is enough FIFO space left to write data for each

active time slot. For example, if one active time slot is running

at an ODR of 100 Hz and a second time slot is decimating by 4

or subsampling at 1/4th the rate of the first time slot for an ODR

of 25 Hz, data is only written to the FIFO at the end of the

sampling period if there is enough room for both active time

slots to write data, regardless of whether the time slot that is

decimating or subsampling is supposed to write data during

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