Using PPS to Synchronize with External GPS

Supported Products

Ladybug5+

Ladybug6

Overview

Pulse per second (PPS) is a common feature in GPS units and IMUs to provide an accurate timing signal to assist with precision synchronization of various devices to a common clock. It is commonly provided as a TTL signal at either 3.3 V or 5 V.

The Ladybug is able to directly accept PPS signals and NMEA data streams over the GPIO pins. This allows the Ladybug to maintain an internal precision time that is continuously synchronized with GPS time. Therefore, the timestamps attached to each image frame may be considered to be within microseconds of GPS time. The Ladybug does not process or output GPS positioning data.

The Ladybug cannot accept an RS232 signal from the GPS device. If your signal is RS232, you need to convert it to TTL for use with this feature.

Note: The Ladybug library has the ability to interface with a GPS device and insert NMEA sentence data into Ladybug images. The data can then be extracted at a later time and be used to generate HTML data, which can be displayed as a Google Map, or KML data, which can be loaded into Google Earth. For information on how to use this feature see Using an External GPS Device with Ladybug Cameras.

Test configuration

Item Description
Camera Ladybug5+
Firmware 1.08.3
Software Ladybug SDK 1.15
GPS device u-blox EVK-M8T

Technical notes

Data formats supported

The camera supports the following NMEA sentence types:

  • GGA—fix information. This is required with either of the following:
  • ZDA—date and time
  • RMC—recommended minimum data for GPS

Timestamp

The LadybugImageInfo contains the timestamp and shutter time information.

In trigger mode, the GPS timestamp is stored at the end of integration. The timestamp accuracy has a positive offset of 45 μs ±8 μs.

In free running mode, the GPS timestamp is stored at the start of integration.

To calculate the mid point of a frame's timestamp:

Mid Point = Timestamp - 45 μs - 0.5 x Shutter Time

GPIO pin assignments

The GPIO pin 4 IO2 is used for GPS data.

The GPIO pin 11 IO3 is used for PPS signal, active on low to high transition.

The GPIO pin 12 is used as a signal ground for IO2 and IO3.

The camera has a 12-pin GPIO connector on the bottom of the case; refer to the diagram below for wire color coding. The GPIO is a Hirose waterproof 12-pin female connector (Mfg P/N:LF10WBP-12SD).

Diagram Color Pin Function Description
Green 1 OPTO_GND Ground for opto-isolated IO pins
Blue 2 I0 Opto-isolated input (default Trigger in)
Brown 3 O1 Opto-isolated output
Orange 4 IO2 Input/Output / GPS data
White 5 +3.3 V Power external circuitry up to 150 mA
Black 6 GND Ground for bi-directional IO, VEXT, +3.3 V pins
Red 7 VEXT Allows the camera to be powered externally
Red 8 VEXT Allows the camera to be powered externally
Red 9 VEXT Allows the camera to be powered externally
Green 10 OPTO_GND Ground for opto-isolated IO pins
Yellow 11 IO3 Input/Output / PPS signal
Black 12 GND Ground for bi-directional IO, VEXT, +3.3 V pins

Opto-isolated electrical characteristics

Values are for reference only and could vary between cameras.

External voltage resistor combinations at lowest frame rate
External
Voltage
External
Resistor
OPTO_OUT 
Low Voltage
OPTO_OUT
High Voltage
Current
3.3 V 200 Ω 1.44 V 3.29 V 8.2 mA
5.0 V 1.0 KΩ 0.88 V 4.94 V 3.8 mA
12 V 2.0 KΩ 1.1 V 11.93 V 5.1 mA
12 V 2.4 KΩ 0.98 V 11.93 V 4.2 mA
24 V 4.7 KΩ 0.75 V 23.75 V 4.5 mA

 

Opto-isolated input output circuit

Opto-Isolated-input-circuit.jpg

Opto-isolated input timing characteristics

krdg4du8.png

Opto-isolated input timing characteristics (5Vpp 1KHz Source Input)

Source

Parameter Symbol Value
Input Low Voltage VL 0.03 V
Input High Voltage VH 4.69 V
Input Threshold High Voltage VTHH 4.22 V
Input Threshold Low Voltage VTHL 0.5 V
Cycle Rise Time tR 93.32 nS
Cycle Fall Time tF> 92.82 nS

FPGA reads

Parameter Symbol Value
Input Low Voltage VL 0.13 V
Input High Voltage VH 3.34 V
Input Threshold High Voltage VTHH 3.02 V
Input Threshold Low Voltage VTHL 0.45 V
Cycle Rise Time tR 17.74 µS
Cycle Fall Time tF 1 µS

Triggering at 15 FPS

Parameter Symbol Opto-isolated
Propagation Delay Low to High (Source POV) tPDLH 556.98 nS
Propagation Delay High to Low (Source POV) tPDHL 26.4 µS
Minimum Positive Pulse Width (Input Source) tMPPW 56.5 µS
Minimum Negative Pulse Width (Input Source) tMNPW 2.16 µS

Opto-isolated output timing characteristics

Opto-isolated output performance at 7.5 FPS (measured with line pull up to Vcc = 5V, Rext = 1.0 KΩ)

Parameter Symbol Opto-isolated
Output Low Voltage VL 0.84 V
Output High Voltage VH 5.06 V
Output Threshold High Voltage VTHH 4.64 V
Output Threshold Low Voltage VTHL 1.35 V
Cycle Rise Time tR 13.06 µS
Cycle Fall Time tF 2.63 µS
Opto Current (when triggering another camera)   3.87 mA
Opto Isolator Delay (High to Low)   2.92 µS
Opto Isolator Delay (Low to High)   24.2 µS 

Non-isolated electrical characteristics

Values are for reference only and could vary between cameras.

Non-Isolated input and output GPIO circuit

Non-isolated-input-output-circuit.jpg

Non-isolated input timing characteristics

Input Performance (measured with a 3.3Vp-p 1KHz Square Wave input @ point A)

Input Source Pulse

Parameter Symbol  
Input Low Voltage VL 0.11 V
Input High Voltage VH 3.34 V
Input Threshold High Voltage (90%) VTHH 3.01 V
Input Threshold Low Voltage (10%) VTHL 0.43 V
Cycle Rise Time tR 154.9 nS
Cycle Fall Time tF 155.32 nS

FPGA reads (measured at point B)

Parameter Symbol  
Input Low Voltage VL 0.6 V
Input High Voltage VH 2.84 V
Input Threshold High Voltage (90%) VTHH 2.62 V
Input Threshold Low Voltage (10%) VTHL 0.82 V
Cycle Rise Time tR  49.37 nS
Cycle Fall Time tF 71.12 nS

Delays (Comparison between point A and point B)

Parameter Symbol  
Propagation Delay Low to High tPDLH 122.88 nS
Propagation Delay High to Low tPDHL 109.15 nS
Positive Pulse Width tMPPW 0.772 mS
Negative Pulse Width tMNPW 32.56 mS

Minimums

Parameter Symbol  
Input High Voltage (point A) VIH ≥ 1.13 V
Propagation Delay Low to High (Between point A and point B) tPDLH 227.46 nS
Propagation Delay High to Low
(Between point A and point B)
tPDHL 108.47 nS
Input Low Voltage (point A) VIL ≤ 1.73 V
Propagation Delay Low to High (Between point A and point B) tPDLH 101.72 nS
Propagation Delay High to Low (Between point A and point B) tPDHL 118.76 nS
Minimum Positive (on) Pulse Width (Input Source, point A) tMPPW 280 nS
Minimum Positive (on) Pulse Width (Camera receives, point B) tMPPW 365 nS
Minimum Negative (off) Pulse Width (Input Source, point A) tMNPW 620 nS
Minimum Negative (off) Pulse Width (Camera receives, point B) tMNPW 553.78 nS

Non-isolated output timing characteristics

GP Output Performance @ Point A (Internal Vcc = 3.3V, Rext = 4.7 KΩ)

Parameter Symbol  
Output Low Voltage VL 0.08 V
Output High Voltage VH 3.12 V
Output Threshold High Voltage (90%) VTHH 2.82 V
Output Threshold Low Voltage (10%) VTHL 0.38 V
Cycle Rise Time tR 10.84 µS
Cycle Fall Time tF 1.08 µS

GP Output Performance @ Point A (adding pull up to Vcc = 5V, Rext = 1 KΩ to the line)

Parameter Symbol  
Output Low Voltage VL 0.31 V
Output High Voltage VH 5.012 V
Output Threshold High Voltage (90%) VTHH 4.54 V
Output Threshold Low Voltage (10%) VTHL 0.78 V
Cycle Rise Time tR 1.99 µS
Cycle Fall Time tF 1.57 µS

Configuring PPS/GPS synchronization

To configure time synchronization you must enable PPS and enable GPS time synchronization.

Using Ladybug API

There are two functions:

  • ladybugSetGpsTimeSync()
  • ladybugGetGpsTimeSync()

These take a camera context and a GpsTimeSyncSettings structure as follows:

  • unsigned int baudRate
  • bool enablePps
  • bool enableGpsTimeSync

Using camera registers

Enable PPS
Offset Name Field Bit Description
0x1EAC PPS Presence_Inq [0] Presence of the feature
0 = not available
1 = available
      [1-30] Reserved
    PPS_Enable [31] Enable PPS
0 = disabled
1 = enabled

 

Set baud rate and enable receive buffer (enable GPS time sync)
Offset Name Field Bit Description
488h SIO_CONTROL_CSR_INQ SIO_Control_Quadlet_Offset [0-31] 32-bit offset of the SIO CSRs from the base address of initial register space
Base + 0h SERIAL_MODE_REG Baud_Rate [0-7] Baud rate setting
0 = 300 bps
1 = 600 bps
2 = 1200 bps
3 = 2400 bps
4 = 4800 bps
5 = 9600 bps (recommended)
6 = 19200 bps
7 = 38400 bps
8 = 57600 bps
9 = 115200 bps
A = 230400 bps
Base + 4h SERIAL_CONTROL_REG RE [0] Enable receive
Indicates if the camera's ability to receive data is enabled. Enabling this register causes the receive capability to be immediately started. Disabling this register flushes the the data in the buffer.
0 = disabled
1= enabled

For a complete list of registers, see the Teledyne Machine Vision Camera Register Reference.

Verifying configuration

You can view the time synchronization status from the LadybugImageInfo:

  • unsigned int ulGpsFixQuality
  • bool bPpsStatus
  • bool bGpsStatus

You can also verify the configuration using camera registers.

Verify time synchronization
Offset Name Field Bit Description
0x1E9C GPS_PPS_TIME_SYNC_VERIFICATION Presence_Inq [0] Presence of the feature
0 = not available
1 = available
      [1-23] Reserved
    GPS_Fix_Quality [24-27] Represents the quality setting from a GGA NMEA sentence
      [28-29] Reserved
    GPS_Signal [30] GPS signal
0 = invalid
1 = valid
    PPS_Signal [31] PPS signal
0 = invalid
1 = valid

API Example

The API example ladybugGPSTimeSync demonstrates the configuration. The example is provided in the Ladybug SDK, available from the downloads page.