ISL29501 Datasheet, PDF (9/23 Pages) Intersil Corporation – Auto gain control mechanism

ISL29501
The AFE in conjunction with the gain from the AGC loop allows for
selection of different detectors in application design. The ADC
relies on the automatic gain control loop to evaluate the optimal
setting for data conversion. This prevents saturation when the
target is at short range.
Functional Overview
The following paragraphs provide additional detail to the function
of the important blocks in the ISL29501. Additional information
may be available in the Related Literature on page 1.
Power Supply Pins
The ISL29501 will operate with a voltage range from 2.7V to
3.3V. There are three power rails: AVCC, DVCC and EVCC. The
AVCC and DVCC supply the digital and analog part of circuits
while the EVCC is dedicated to the emitter driver section.
Intersil recommends decoupling the analog and digital supplies
to minimize supply noise. Noise can be random or deterministic
in nature. Random noise is decoupled like in any other system.
Synchronous noise (4.5MHz) is seen as crosstalk by the chip and
directly affects distance measurements. Crosstalk calibration
will mitigate this but it is better to target this frequency directly,
particularly on EVCC.
Power-On Reset
When power is first applied to the DVCC pin, the ISL29501
generates an internal reset. The reset forces all registers to their
default values and sets the sequencer to an initial state.
Chip Enable (CEn) Pin
The CEn pin is an active low input pin. When asserted (pulled low),
the device will bias the internal circuit blocks, band gaps, references
and I²C interface. Once CEn is enabled writing 0x01 b0 = ‘1’ will
disable the chip. It can be re-enabled by writing it back to ‘0’
providing CEn stays low. This allows software control. Register 0x01
defaults to 0 or enabled.
Changing chip enable does not alter register values.
Sample Start (SS) Pin
Sample Start (SS) pin is an input logic signal, which triggers a
measurement cycle. This signal is needed to start measurements
in free run mode and for each measurement in single shot mode.
If a trigger is received during an active measurement the request
is ignored.
Command Register
The command Register (0xB0) allows the user to operate under
software control. There are 3 commands that each perform
useful functions without hardware interaction.
• A soft-start can be initiated by writing 0xB0 to 0x49. This
register bit emulates the single shot pin. This acts like the SS
pin to start measurements.
• A soft-reset can be initiated by writing 0xB0 to 0xD7. This
action resets all registers to power-on default values with the
exception of brownout bit. This function performs similar to the
power cycle.
• A soft-clear can be initiated by writing to 0xB0 to 0xD1, this
stops all conversions and resets the accumulators and the
sensor will stop if it is in continuous mode. This function is a
sequencer reset.
Interrupt (IRQ) Pin
The ISL29501 can be configured to generate interrupts at the
completion of a sensing cycle. This allows the host to perform
other tasks while a measurement is running.
The IRQ pin is an active low output logic pin. It is an open-drain
output pin and requires a pull-up resistor to DVCC.
The host should service the IRQ request by reading the 0x60
register. The register is cleared upon reading (self clear). If the
sensor is set to signal sample mode, then the sensing stops and
awaits for the next sample start signal.
If the ISL29501 is set to continuous mode, then the IC will begin
the next sensing sample according to the preconfigured
sampling time period.
Sampling Modes
The ISL29501 provides two operating sample modes; single shot
mode and continuous mode.
TABLE 1. Reg0x13 SAMPLING MODES
BIT
REGISTER PREFERENCE
MODE OF OPERATION
0x13[0]
0
Continuous sampling
0x13[0]
1
Single shot sampling
Single Shot
In single shot mode one measurement is made. The sampling
period is normally not important since the MCU is controlling
each measurement. The duration of the measurement is
controlled by the integration time and the MCU latency. This
allows the greatest flexibility of the measurement duty cycle and
therefore the power consumption.
Continuous Mode
Continuous mode operation is used for systems where the sensor
is continuously gathering data at a predefined integration and
sampling period using the internal timing controller. This is the
chip default.
The data is available to the host after every sample period and
the sensor will keep operating in this mode until changed by the
MCU. If the interrupt is enabled the IRQ pin will toggle after each
measurement.
By adjusting the sample period and the sample period range
(Registers 0x11 and 0x12) over 3.5 seconds between
measurements is possible. For measurement intervals greater
than this single shot mode must be used.
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FN8681.3
June 29, 2016