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Texas Instruments Programming Chirp Parameters in TI Radar Devices Application notes
Application Report
SWRA553 – May 2017
Programming Chirp Parameters in TI Radar Devices
Vivek Dham
ABSTRACT
This application report provides information on how to select the right chirp parameters in a fast FMCW
Radar device based on the end application and use case, and program them optimally on TI’s radar
devices.
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Contents
Introduction ................................................................................................................... 2
Impact of Chirp Configuration on System Parameters .................................................................. 2
Chirp Configurations for Common Applications.......................................................................... 7
Configurable Chirp RAM and Chirp Profiles.............................................................................. 7
Chirp Timing Parameters ................................................................................................... 8
Advanced Chirp Configurations .......................................................................................... 11
Basic Chirp Configuration Programming Sequence ................................................................... 12
References .................................................................................................................. 14
List of Figures
1
Typical FMCW Chirp ........................................................................................................ 2
2
Typical Frame Structure .................................................................................................... 2
3
Angle Estimation Basics .................................................................................................... 5
4
Example Antenna Gain Pattern ............................................................................................ 5
5
Effective Recevier Channels in MIMO Radar ............................................................................ 6
6
Chirp and Profile RAM Memory Allocation ............................................................................... 8
7
Example Usage of the Ramp Timing Calculator ....................................................................... 10
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Sub Frame Structure Showing Three Bursts Looped Twice (start index having an offset of 2) ................. 11
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Example of Advanced Frame Configuration of Two Sub Frames ................................................... 12
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Radar Configuration Sequence ........................................................................................... 12
List of Tables
1
Example Chirp Configurations for Typical Applications................................................................. 7
2
Chirp Timing Parameters ................................................................................................... 8
3
Typical Synthesizer Ramp Down Times for Different Modulation Bandwidths ...................................... 9
4
Typical IF/DFE Filtering Latencies as a Function of DFE Mode and Output Sampling Rate
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1
Introduction
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Trademarks
All trademarks are the property of their respective owners.
1
Introduction
Frequency Modulated Continuous Wave (FMCW) mmWave radar sensors are becoming increasingly
popular for multiple automotive and industrial applications. The system requirements and care-abouts in
each of these applications could be very different. Range requirement, range resolution, max velocity
requirement, sensor field of view, data memory, processor MIPS, and so forth are some of the aspects
that need to be analyzed based on the end application. Understanding the relationships between the
FMCW chirp configuration and system performance parameters helps in selecting the right chirp
configurations.
TI’s mmwave radar devices (MMIC) provide large flexibility in configuring chirp parameters and also allow
multiple chirp configurations in a single frame. The timing parameters are accurately controlled by the
digital timing engine and a built-in radio processor without heavy real-time software interference. This
document describes the programming of chirp parameters and explains the various system considerations
that determine the values for these parameters.
2
Impact of Chirp Configuration on System Parameters
In linear FMCW radars, the transmit (TX) signal is a single tone with its frequency changing linearly with
time. This sweep in frequency is commonly referred to as a “chirp”. A set of these chirps form a “Frame”
and this can be used as the observation window for the radar processing. The various parameters of the
chirp ramp (like frequency slope, sweep bandwidth, and so forth) impact the system performance.
Figure 1 depicts a single chirp and the associated timing parameters. Figure 2 shows frame structure that
consists of a series of chirps followed by inter frame time. This represents ‘Fast FMCW’ modulation, where
each chirp is typically 10’s of µs in duration.
Figure 1. Typical FMCW Chirp
Figure 2. Typical Frame Structure
2
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The following sections list key system performance parameters that are typically considered in any radar
application and how the chirp configuration impacts each one of them.
2.1
Measurement Range and Range Resolution
The maximum and minimum distance over which a radar sensor can detect objects is an important
parameter for a radar sensor. Also, the range resolution (ability to distinguish two nearby objects) is
another important metric.
2.1.1
Maximum Range
In applications like automotive adaptive cruise control (ACC), it’s important to be able to view a far off
object (>150m). Detecting a far-off object can be limited by either the SNR of the received signal or the IF
bandwidth supported by the Radar device.
The max range relationship with the IF bandwidth is shown in Equation 1. TI’s AWR1243 radar device
provides a large 15 MHz bandwidth, allowing more flexibility in the slope that can be used, which indirectly
helps increase the max velocity as will be seen later.
IFmax u c
Rangemax
2uS
(1)
IFmax → Maximum IF bandwidth supported
c → Speed of light
S → Slope of the transmitted chirp
Note that the IFmax is also dependent on the ADC sampling frequency (ADCsampling) used. In a complex 1x
sampling mode, the IF bandwidth is limited to 0.9* (ADCsampling). In case of complex 2x and real sampling
modes, the IF bandwidth is limited to 0.9* (ADCsampling)/2 . The maximum ADC sampling frequency in the
TI’s radar devices is 37.5 Mhz.
The other aspect that could limit the max range is the signal to noise ratio (SNR) of signal received by the
receiver. This depends on:
• RF performance of the Radar device, like TX output power, RX noise figure, as well as chirp
parameters like chirp duration and number of chirps in the frame.
• Antenna parameters like the TX and RX antenna gain in the direction of interest.
• Object characteristics like Radar Cross Section (RCS). RCS is a measure of the amount of energy the
object reflects back. This decides how detectable the object is with a radar sensor.
• Minimum SNR required by the detection algorithm to detect an object.
Rangemax based on SNR
4
PtGRx GTx c 2 VNTr
fC2 4S
3
u kT u NF u SNRdet
(2)
Pt → Tx output power
GRx ,GTx → RX and TX Antenna gain
σ → RCS of the object
N → Number of chirps
Tr → Chirp time
NF → Noise figure of the receiver
SNRdet → Minimum SNR required by the algorithum to detect an object
k → Boltzman constant
Tdet → Ambient temperature
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Impact of Chirp Configuration on System Parameters
2.1.2
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Range Resolution
In many applications it is important to be able to resolve two closely spaced objects as two separate
objects, rather than detect them as one. The smallest distance between two objects that allows them to be
detected as separate objects is referred to as range resolution. This primarily depends on the chirp sweep
bandwidth that the radar sensor can provide. The larger the sweep bandwidth, the better the range
resolution. TI’s radar devices support a 4 GHz sweep bandwidth that allows a range resolution of as low
as approximately 4cm.
c
Rangeresolution
(3)
2uB
c → Speed of light
B → Sweep bandwidth of FMCW chirp
Better range resolution also helps in detecting very close by objects, hence, improving a minimum
detection range.
2.2
2.2.1
Measurement Velocity and Velocity Resolution
Maximum Velocity
Along with the distance, the relative velocity of the object is another critical parameter of interest. The
maximum measurable velocity in Fast FMCW modulated radars depends on the chirp cycle time, that is,
the time difference between the start of two consecutive chirps. This in turn depends on how fast the
frequency sweep can be performed and the minimum inter-chirp time allowed.
The faster the MMIC can ramp the frequency, the higher the maximum unambiguous velocity. TI’s MMIC
allows a fast ramp of 100 MHz/µs. Also the closed loop PLL is designed to support a very fast settling of
the frequency ramp. Hence, the time taken for the VCO to jump from the end of the ramp frequency to
restart the next ramp is very low and allows for a smaller idle time (as low as 2 µsec). For minimum idle
time computation, see Section 5.
,
Unambiguousmax velocity
4TC
(4)
Tc → Total Chirp time ,which inlcudes chirp time+idle time
ƛ → Wavelength of the signal used
The actual measurable max velocity can be extended beyond the unambiguous max velocity using higher
level algorithm.
2.2.2
Velocity Resolution
In applications, like park assist, you might need to separate out objects with small velocity differences, for
which good velocity resolution is needed. Velocity resolution mostly depends on the transmit frame
duration, that is, increasing the number of chirps in a frame improves the velocity resolution.
,
Unambiguousmax velocity
2NTC
(5)
Nc → Number of Chirp in a frame
2.3
Angular Range and Resolution
In order to locate the object in the 2D space, the angle of the object is also required along with the
distance. In a radar system, the angle is estimated by receiving the reflected signal from the object using
multiple receivers that are spaced apart with a distance ‘d’. The signal arriving at each of the successive
2S d sin T
4
receivers is delayed by d*sin(θ) and this “delay” causes a phase shift of
O
between each of the receivers is used to estimate the angle (θ) of the object.
. This phase shift
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Figure 3. Angle Estimation Basics
The measurable unambiguous angular field of view from the MMIC perspective depends on the spacing
between the receivers (d).
Max unambiguous angular range
§ ·
sin 1 ¨ ã ¸
© 2d ¹
(6)
d → Spacing between receiver antennas
ƛ → Wavelength
So for the widest angular field of view, the spacing of the receiver antenna should be ƛ/2 , theoretically
giving ±90° viewing range.
Apart from the antenna spacing, the measurable distance at different angles would also depend on the
antenna gain pattern. Typically antennas would have a peak gain at one angle (mostly at 0°, that is,
directly facing the front of the antenna) and then the gain would reduce as the angle increases. Figure 4
shows an example antenna pattern where the gain at 90° angle is > 15 dB lower than what it is at 0°
angle.
Figure 4. Example Antenna Gain Pattern
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Impact of Chirp Configuration on System Parameters
2.3.1
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Angular Resolution
Apart from the angular field of view, it might also be important to resolve two objects at close by angles,
that is, have good angular resolution. For example, in an automotive radar use case, it would be important
to detect two cars far off in two different lanes rather than detect them as one single car. In general, the
angular resolution measurement depends on the number of receiver antennas available. The larger the
number of antennas, the better the resolution.
180
O
Angular resolution (deg)
u
d u NRX cos T
S
(7)
θc → Angle of interest, that is, the angle at which the objects are present
NRx → Number of receiver antennas
The angular resolution can be further improved using multiple transmitters. For more details, see MIMO
Radar. If there are multiple transmitters available, then the transmit antennas can be spaced in such a
way that each of the transmitters paired with the set of receivers together create a virtual receive array.
For example, if there are 3 TX and 4 RX, then a MIMO radar system can produce the equivalent angular
resolution of 12 virtual channels.
O
180
u
Angular resolution
S
d u NRXNTX cos T
(8)
NTx → Number of transmit antennas
Figure 5. Effective Recevier Channels in MIMO Radar
6
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Chirp Configurations for Common Applications
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3
Chirp Configurations for Common Applications
The most common applications for radar in the automotive context include short range radar (typically
mounted in the corners) and mid or long-range radar (typically front facing).
This section shows chirp configurations for a 22m USRR, 45m SRR, 125m MRR and a 225m LRR usecase. It is important to note that these are just generic example configurations and it is possible to change
the parameters based on the customer’s specific system performance requirements. Sample chirp
configurations and use-cases as applicable to various TI’s mmwave radar devices are shown in the
device-specific overview white papers.
Table 1. Example Chirp Configurations for Typical Applications
Parameter
Max unambiguous
range
Sweep bandwidth
Ramp slope
Inter-chirp duration
Units
LRR
MRR
SRR
USRR
m
225
125
45
22.5
MHz
300
540
750
1500
MHz/us
10
12
15
30
us
8
10
12
50
Number of chirps
-
256
128
128
128
Range resolution
m
0.50
0.28
0.20
0.1
Chirp duration
us
30
45
50
50
Max umambiguous
relative velocity (1)
kmph
92.28
63.75
56.56
35.3
Max beat frequency
MHz
15
10
4.5
4.5
ADC sampling rate
(complex)
Msps
16.67
11.11
5.00
5.00
500
500
250
250
Number of samples
per chirp
Range FFT size
-
512
512
256
256
Frame time (total)
ms
9.728
7.04
7.94
12.8
Frame time (active)
ms
7.68
5.76
6.4
6.4
Radar data memory
required
KB
2048
1024
512
512
(1) The maximum velocity can be increased beyond the max unambiguous velocity by using higher level algorithms.
4
Configurable Chirp RAM and Chirp Profiles
The TI radar devices allow you to control the parameters of chirps in a frame by defining chirp profiles,
and variations on top of these profiles through a chirp configuration RAM. Chirp profiles are basic chirp
timing templates, useful in defining chirp variants with significant differences in one or more defining
parameters (start frequency, slope, idle time, and so forth). The radar devices allow you to program four
different chirp profiles. In addition, up to 512 unique chirps can be pre-programmed and stored in the chirp
configuration RAM. Each chirp definition entry in the RAM can belong to one of the four profiles, and can
optionally differ from their parent profile by small dithers in some of the parameter values. A frame would
then consist of a sequence of chirps from a start index to an end index in the chirp configuration RAM
which can be looped over up to 255 times.
The parameters that are controllable per profile are:
• Start frequency
• Frequency slope
• Idle time
• ADC start time
• Ramp end time
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Chirp Timing Parameters
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Each chirp in the chirp configuration RAM can have small dither values that are added to the profile
parameters defined in the profile RAM. These are:
• Start frequency variable
• Frequency slope variable
• Idle time variable
• ADC start time variable
The chirp RAM and profile RAMs are depicted in Figure 6.
Figure 6. Chirp and Profile RAM Memory Allocation
5
Chirp Timing Parameters
Table 2 provides a list of the parameters that govern the chirp timing. It also shows the relationship of
these parameters to form a chirp.
Table 2. Chirp Timing Parameters
Sl. No. Parameter name
Description
1
Idle time
The time between the end of previous chirp and start of next chirp.
2
TX start time
The time from the start of the ramp at which transmitter is turned on.
3
ADC start time
The time from the start of the ramp when the ADC starts sampling the data.
4
Ramp end time
The time from the start of the ramp until the chirp continues ramping. After this time the synthesizer
frequency is reset to the start frequency of the next chirp.
The following sections list the system considerations that need to be taken to determine the value for each
of these parameters. The actual numbers for each of these parameters can be derived using a chirp
timing parameter configuration calculator utility in Radar Studio application available as part of the DFP
package.
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5.1
Idle Time
The minimum required idle time is determined predominantly by the synthesizer ramp down settling time,
which is a function of ramp down bandwidth. The synthesizer ramp down settling times for some typical
bandwidths are tabulated in Table 3.
Table 3. Typical Synthesizer Ramp Down Times for Different Modulation Bandwidths
Ramp Bandwidth
Synthesizer Ramp Down Time (µs)
< 1 GHz
2
> 1 GHz and < 2 GHz
3.5
> 2 GHz and < 3 GHz
5
> 3 GHz
7
Table 3 can directly be used to set the minimum idle time, for cases where the ADC sampling rate is 5
Msps or higher. However, for low sampling rates (< 5 Msps), there is also another constraint that needs to
be kept in mind when programming the minimum idle time. This comes from digital pipeline delays in the
sigma-delta ADC decimation chain. Due to this, the minimum idle time might need to be slightly higher
than the values shown in Table 3. The chirp parameter configuration calculator utility (part of the Radar
Studio tool, www.ti.com/tool/mmwave-dfp) takes care of this constraint as well and provides the overall
recommended idle time.
5.2
ADC Start Time
The ADC start time does NOT have a hard minimum requirement for correct chip functionality. However,
there is a trade-off between the value of this parameter and the quality of “beginning of chirp” signal
envelope settling, dependent on the following factors:
• Synthesizer PLL ramp-up settling time, which is a function of ramp slope
• HPF step response settling, which is a function of HPF corner frequencies
• IF/DFE LPF settling time, which is a function of DFE output mode and sampling rate.
The chirp timing parameter configuration calculator utility takes these aspects into account in computing
the recommended value.
The user may also have other considerations such as maximum round-trip delay of reflections from the
target based on the end use-case that may be added to the recommended ADC start time from the utility.
The customer may also choose to use a smaller value than the recommended value, in order to reduce
the total inter-chirp gap, by trading off the quality of settling.
5.3
Ramp End Time
The ramp end time is the sum of (a) the ADC start time, (b) the ADC sampling time and (c) the excess
ramping time at the end of the ramp. There is no hard requirement on excess ramping time. However,
there is again a trade-off between the amount of excess ramping and the “end of chirp” signal envelope
settling performance. The primary factor determining the excess ramping required is IF/DFE filtering
latency, which is again a function of DFE output mode and sampling rate.
The chirp timing parameter configuration calculator utility takes these aspects into account in arriving at
the recommended value for this parameter.
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Chirp Timing Parameters
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Table 4. Typical IF/DFE Filtering Latencies as a Function of DFE Mode and Output Sampling Rate
DFE Mode
DFE Sampling Rate (Msps)
IF/DFE Filtering Latencies (µs)
Complex1x
16.7
0.3
10
0.5
Real/Complex2x
5.4
5
1.0
33.3
0.1
16.7
0.2
10
0.4
5
0.8
Example Timing Calculation Using the Calculator
The Ramp timing calculator available in the Radar Studio tool (part of the DFP package:
(www.ti.com/tool/mmwave-dfp) provides the right timing parameter value recommendations based on the
user configurations (slope, ADC samples, sampling rate). These recommendations are based on the
minimum requirements from TI’s radar device.
Figure 7. Example Usage of the Ramp Timing Calculator
You may have further considerations over these based on the specific use case. These considerations
need to be accounted for separately.
For example, if the raw ADC data is being set over the CSI or LVDS path, the idle time setting needs to
make sure the transfer on the CSI/LVDS interface is complete before the next chirp data is available. The
time available for this data transfer is = “ramp end time” + “idle time”. In case of CSI, the overhead time
due to the MIPI protocol also needs to be accounted for.
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Advanced Chirp Configurations
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Advanced Chirp Configurations
6.1
Multi-Mode Radar Applications
As you have seen that based on the desired application the chirp configurations need to be set differently.
But what if we need to support multiple modes, for example short range and mid-range, simultaneously
using a single radar device? The advanced frame configuration available in TI’s radar allows for large
flexibility to have multiple chirp configurations in a single frame. The frame can be constituted using a
sequence of “sub-frames” with each of these sub-frames representing a different radar mode. The
rlSetAdvFrameConfig API helps enable this kind of configuration. For details, see the “AWR1XXX Radar
Interface Control Document” in the DFP package: www.ti.com/tool/mmwave-dfp).
To provide maximum flexibility of the chirps within a frame the advanced frame config API provides the
ability to break a frame into different sub frames (up to 4). Each of the sub frames consists of multiple
bursts of chirps (up to 512 bursts) . Each burst can consist of 512 unique chirps which are associated to
one of the 4 profiles, and the start chirp index can be programmed to have a fixed offset from the previous
burst. A set of bursts in a sub-frame can be further looped in software up to 64 times. The below figure is
an example how a sub frame is formed.
Figure 8. Sub Frame Structure Showing Three Bursts Looped Twice (start index having an offset of 2)
The frame is formed by a maximum of four such sub frames, and each of the sub frames can have a
different set of chirps. The different chirps can also use different transmitters (which could possibly have
different antenna configurations). Figure 9 shows an example of the different chirp profiles and sub frames
that can form a frame.
The timing requirements for the advanced frame is as follows: Inter-burst time should be ≥ 50 µsec, inter
sub-frame time should be ≥ 100 µsec and inter frame time should be ≥ 200 µsec.
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Advanced Chirp Configurations
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Figure 9. Example of Advanced Frame Configuration of Two Sub Frames
6.2
Individual Transmitter Binary Phase Modulation (BPM)
TI’s radar device allows each of the transmitter outputs to be modulated by a phase of 0° or 180°. This
allows usage of multiple transmitters simultaneously by using mutually uncorrelated binary codes. hence,
improving the effective SNR.
The phase configuration is done by the “rlSetBpmChirpConfig” API. For details on BPM and MIMO radar,
see MIMO Radar .
7
Basic Chirp Configuration Programming Sequence
TI’s mmwave link APIs provide simple interface to the MMIC and offer full flexibility to configure all the
chirp parameters based on the application and requirement. The details of each of the APIs and the
parameter descriptions are available in the “AWR1XXX Radar Interface Control Document:
www.ti.com/tool/mmwave-dfp). Figure 10 shows a typical sequence to be followed to configure the chirp
and frame.
Figure 10. Radar Configuration Sequence
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7.1
Basic Chirp Configuration Programming Sequence
Device Manager APIs
These APIs are used to power on and initialize the sensor:
• rlDevicePowerOn
This function initializes the driver and does the necessary resource allocation for the driver. It initializes
the host protocol driver by creating the necessary OS services like semaphore, mutex, queues and so
forth. It also brings radar devices (multiple devices in case of cascade) out of reset and opens the
communication channel (SPI, Mailbox, and so forth) with these devices.
• rlDeviceRfStart
This function initializes the RF (BIST) subsystem in the radar device. The function returns immediately
and RF initialization completion is indicated by the asynchronous event
(RL_EVENT_AR_DEVICE_START_COMPLETE). User application should wait for this event before
invoking any Radar Sensor Control APIs.
• DeviceFileDownload
This function downloads a binary file from the host to the internal RAM of the radar device. This file
could be a firmware patch file, application code, calibration data or configuration data.
7.2
Radar RF Control APIs
These APIs are used to configure the RF parameters, chirp profiles and frame configurations.
• rlSetChannelConfig
This API allows configuring of the number of TX antennas (out of 3) and number of RX antennas (out
of 4) to be used. It also allows the selection of whether they are using the sensor in standalone mode
or cascade mode.
• rlSetAdcOutConfig
This API allows configuring of the number of bits per sample (12/14/16). It also allows the choice of
whether the ADC data should be real only, Complex -1x or Complex- 2x.
• rlSetLowPowerModeConfig
This API allows the setting of a low power ADC mode for power saving. In this mode, the maximum
ADC sampling rate is limited.
• rlSetProfileConfig
This API allows configuring the chirp “profile”, which defines a template for a chirp. These
configurations include chirp start frequency, idle time, ADC start time, chirp slope, chirp duration, TX
power level in the chirp, number of ADC samples per chirp, the ADC sampling rate, High pass filter
(HPF) cutoff frequencies and RX gain setting. Up to four profiles can be defined and in a particular
frame (which can have up to 512 unique chirps) the chirps in the frame can belong to any of these four
profiles. Since a single profile may not suffice all the intended use cases of the sensor, having flexibility
to have multiple profiles allows usage of a single sensor in multiple scenarios/use cases.
• rlSetChirpConfig
Once the profiles are defined, each of the unique chirps can be associated to one of these profiles.
Apart from this, the API also allows limited chirp to chirp variations (beyond the profiles) in some of the
important parameters like start frequency, idle time and ADC start time. The API also allows selecting
the transmitters to be used in that particular chirp.
• rlSetFrameConfig
This API allows selecting the sequence of the chirps that form the frame, number of frames that need to
be transferred and the periodicity of the frames. The periodicity would define the inter-frame time
(periodicity – chirptime), hence, the duty cycle of the transmission. The frame could be software API
triggered (via rlSensorStart API ) or externally triggered using the SYNC_IN signal. In case of the HW
trigger option, a programmable delay can be set from the SYNC_IN edge.
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Basic Chirp Configuration Programming Sequence
7.3
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Radar Data Control APIs
The ADC data captured during the chirps is transferred out of the device on the high speed debug
interface or CSI. The data control APIs allow configuring the ADC data that needs to be transferred out
and the high speed interface (LVDS/CSI) configurations. Along with the ADC data, some additional
information related to the quality of the chirp and chirp parameters (that are referred to as CQ (chirp
quality) and CP (chirp parameter), respectively) can also be transferred out. The LVDS/ CSI
configurations, lane configurations, and so forth can be done using these APIs. For details on these APIs
and the parameters, see the “AWR1XXX Radar Interface Control Document” (part of the DFP package:
www.ti.com/tool/mmwave-dfp).
7.4
Frame Trigger API
Once the chirps and frames are configured, they can be triggered either via a software API or hardware
triggered using the digital SYNC_IN signal. The software API to trigger the frame is rlSensorStart.
8
References
MIMO Radar
14
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