Lime-GPSDO v1.0 hardware description

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Lime-GPSDO Board Key Features

The Lime-GPSDO development board provides high stability clock source for timing sensitive applications. Clock frequency is being constantly monitored inside MAX10 FPGA and tuned by PPS signal from GNSS module.

Lime-GPSDO v1.0
Lime-GPSDO v1.0

For more information on the following topics, refer to the respective documents:

  • MAX10 device family, refer to Intel documentation link
  • N20B GNSS module resources link

Lime-GPSDO board features:

  • USB Interface
    • Silicon labs USBXpress Family USB-to-UART bridge CP2102N.
  • FPGA Features
    • MAX10 10M16SAU169C8 device in 169-pin UBGA
    • 16K logic elements
    • 549 Kbits embedded memory (M9K) and 2368 Kbits of user Flash Memory
    • 45 embedded 18x18 multipliers
    • 1 PLLs
  • FPGA Configuration
    • JTAG mode configuration
  • Memory Devices
    • 4Mbit FLASH
    • 128Kbit (16K x 8) EEPROM
  • Connections
    • microUSB2.0 (type B)
    • SMA connectors for clock IN/OUT, time pulse output and GNSS antenna
    • FPGA GPIO header (0.05” pitch)
    • FPGA JTAG connectors (0.05” pitch and side connector)
    • 5V DC power jack and pinheader
    • Backup battery connector for GNSS receiver
    • Clock output pinheader
    • External UART connector
    • External I2C connector
  • Clock System
    • 30.72MHz VCOCXO:
      • Frequency calibration ±0.5ppm;
      • Frequency stability over temperature in still air ±20ppb;
      • Frequency slope ΔF/ΔT in still air ±1.2ppb/°C
    • Possibility to tune VCOCXO by onboard DAC
    • Possibility to use GNSS PPS signal as a reference when tuning VCOCXO frequency
  • Board Size without connectors 50.50mm x 80mm (1.99” x 3.15”)

Board Overview

Lime-GPSDO board version 1.0 picture with highlighted major components and connections presented in Figure 1 and Figure 2. There are three connector types – data and debugging (USB2.0, FPGA GPIO and JTAG), power (DC jack and external supply pinheader), clock source output, GNSS antenna and time pulse output.

Figure 1 Lime-GPSDO top side components and connectors
Figure 2 Bottom side components


Board components description listed in the Table 1 and Table 2.

Table 1. Board components
Featured Devices
Board reference Type Description
IC5 GNSS module GNSS receiver
IC5 FPGA Intel MAX 10 (10M16SAU169C8G 169-UBGA)
IC9 USB UART Silicon labs USB-to-UART bridge
Miscellaneous devices onboard
IC13 IC Temperature sensor LM75
Configuration, Status and Setup Components
J9 JTAG chain pin header FPGA programming pin header for Altera USB-Blaster download cable, side connection.
J10 JTAG chain pin header FPGA programming pin header for Altera USB-Blaster download cable, 0.05” pitch
LEDS1, LEDS2, LEDS3 Red-green status LEDs User defined FPGA indication.
General User Input/Output
J12 Pin header 8 FPGA GPIOs plus 2 power pins, 0.05” pitch
SW1 Slide switch 4 slide switches connected to FPGA
SW2 Push button Push button connected to FPGA
Memory Devices
IC11 EEPROM 128K (16K x 8) EEPROM connected to FPGA
IC10 IC I²C EEPROM Memory 128Kb (16K x 8), connected to FPGA I2C bus
IC7 Flash memory 4Mbit FLASH memory connected to FPGA
Communication Ports
J11 USB2.0 connector microUSB2.0 (type B) connector
J8 Header GNSS USB 2.0 connection header (unused)
J15 Header External UART interface header, can be used to control external periphery. Additional logic has to be implemented in FPGA.
J16 Header External I2C interface header, can be used to control external periphery. Additional logic has to be implemented in FPGA.
Clock Circuitry
XO1 VCOCXO 30.72MHz voltage-controlled crystal oscillator used as a reference clock.
XO2 XO 50MHz crystal oscillator, used for FPGA logic.
IC2 DAC Analog devices 16bit Digital-to-analog converter for VCOCXO voltage control
J1 Pin header Optional clock output of XO1
J2 Pin header Optional Voltage control input for XO1
J3 SMA connector Time pulse output from GNSS modules
J5 SMA connector Reference clock output
J6 U.FL connector Reference clock output, connector is not fitted by default.
Power Supply
J14 DC input jack External 5V DC power supply
J13 Pin header External 5V DC power supply and main internal power rail
Other
J4 SMA connector Antenna input for GNSS module
J7 Pin header Backup battery connection header for GNSS module. Typical 3.0V (follow recommendation in fitted GNSS module datasheet)

LimeSDR-Mini Board Architecture

The heart of the Lime-GPSDO board is Intel MAX10 FPGA. Its main function is to measure VCOCXO clock frequency and tune it by PPS signal from GNSS module and provide control. The block diagram for Lime-GPSDO board is presented in the Figure 3.

Figure 3 Lime-GPSDO Development Board Block Diagram

GNSS module

GNSS module has serial data communication interface, timing pulse and other control signals connected to FPGA. It also has SMA connector for external antenna.

Table 2. GNSS module connection
Chip pin (IC5) Chip reference (IC5) Schematic signal name FPGA pin FPGA I/O standard Comment
3 TIMEPULSE GNSS_TPULSE A2 3.3-V LVCMOS Also connected to J3 SMA conn.
4 EXTINT GNSS_EXTINT B1 3.3-V LVCMOS
8 RESET_N GNSS_RESET C1 3.3-V LVCMOS
14 LNA_EN /RESV GNSS_OFF F1 3.3-V LVCMOS
15 RTK_STAT /RESV GNSS_ANT_DET E1 3.3-V LVCMOS
16 GEOFENCE_STAT /RESV GNSS_ANT_OK D1 3.3-V LVCMOS
18 DDC_SDA /SPI_CSN GNSS_DDC_SDA B2 3.3-V LVCMOS
19 DDC_SCL /SPI_CLK GNSS_DDC_SCL A3 3.3-V LVCMOS
20 UART_TX /SPI_MISO GNSS_UART_TX B3 3.3-V LVCMOS
21 UART_RX /SPI_MOSI GNSS_UART_RX A4 3.3-V LVCMOS
11 RF_IN RF_IN - - Connected to J4 SMA conn.

By default time pulse output (J3 SMA connector) is connected to FPGA pin C2 output but can be changed to GNSS time pulse signal by removing R8 resistor and soldering R9 resistors. Refer to Figure 4.


Figure 4 Time pulse output selection

GPIO

There are eight general purpose input/output pins from FPGA connected to FPGA GPIO header (J12). Schematic names and pin connections can be found in Table 3.

Table 3. GPIO connection
Connector pin Schematic signal name FPGA pin FPGA I/O standard Comment
1 FPGA_GPIO0 M12 3.3V
2 FPGA_GPIO1 M10 3.3V
3 FPGA_GPIO2 N12 3.3V
4 FPGA_GPIO3 N10 3.3V
5 FPGA_GPIO4 M11 3.3V
6 FPGA_GPIO5 M9 3.3V
7 FPGA_GPIO6 N11 3.3V
8 FPGA_GPIO7 N9 3.3V
9 GND -
10 VCC - 3.3V or 5V selectable power rail.


Voltage for pin 10 of J12 connector can be 3.3V (default) or 5V. To connect this pin to 5V power rail remove R47 and solder R48 resistors (see Figure 5).

Indication LEDs

Lime-GPSDO board comes with four dual color (red and green (RG)) indication LEDs. Most of LEDs are connected to FPGA and their function can be changed. Default LEDs functions and other information are listed in the table below.


Table 4. Default LEDs functions
Board Reference Schematic signal name Board label FPGA pin Description
LED1 FPGA_LED1_G LED1 A11 GNSS lock and VCOCXO tune state:
Solid RED – no GPS lock;
Blinking RED – GPS is locked, 1s lowest accuracy tune state;
Blinking RED/GREEN - GPS is locked, 10s accuracy tune state;
Blinking GREEN - GPS is locked, 100s highest accuracy tune state.
FPGA_LED1_R B11
LED2 FPGA_LED2_G LED2 A10
FPGA_LED2_R B10
LED3 FPGA_LED3_G LED3 A8
FPGA_LED3_R A9
LED4 - LED4 VCC3P3 - Green LED indicates VCC3P3 power rail presence. Red LED is unused
- -

Switch and push button

There are four sliding switches and one push button connected to FPGA. Sliding switch to “ON” position sets logic “0” level and opposite sets logic “1”. Default functions and connection information are listed in the table below.

Table 5. Default LEDs functions
Board Reference Schematic signal name Board label FPGA pin Description
SW1 FPGA_SW0 FPGA SWITCH N7 FPGA_SW[1:0]=x0 - UART of GNNSS module connected to USB UART
FPGA_SW[1:0]= 01 - limegnss_gpio module connected to USB UART
FPGA_SW1 M7
FPGA_SW2 N8 Not used
FPGA_SW3 M8 Not used
SW2 FPGA_BTN FPGA BTN B7 Board reset

Communication interfaces

Lime-GPSDO board has various options of communication interfaces. There is USB-to-UART bridge and micro USB connector, pin headers for UART, I2C and interfaces for onboard periphery. For graphical representation see Figure 6 and detailed description can be found in Table 7.

Figure 6 Communication interfaces
Table 6. Communication interface pins
Schematic signal name FPGA pin I/O standard Comment
FPGA_I2C_SCL J1 3.3V Serial Clock
FPGA_I2C_SDA K1 3.3V Data
EXT_I2C_SCL B4 3.3V Serial Clock
EXT_I2C_SDA A5 3.3V Data
FPGA_SPI1_SCLK N5 3.3V Serial Clock (FPGA output)
FPGA_SPI1_MOSI N4 3.3V Data, master output
FPGA_SPI1_DAC_SS N6 3.3V Slave select
FPGA_SPI2_SCLK M4 3.3V Serial Clock (FPGA output)
FPGA_SPI2_MOSI N3 3.3V Data, master output
FPGA_SPI2_FLASH_SS N2 3.3V Slave select
GNSS_UART_TX B3 3.3V GNSS UART transmit (FPGA input)
GNSS_UART_RX A4 3.3V GNSS UART receive (FPGA output)
CP_CTS F13 3.3V Clear To Send control input (FPGA output)
CP_RTS F12 3.3V Ready To Send control output (FPGA input)
CP_RXD G13 3.3V Asynchronous data input (FPGA output)
CP_TXD G12 3.3V Asynchronous data output (FPGA input)
CP_DSR H13 3.3V Data Set Ready control input (FPGA output)
CP_DTR J13 3.3V Data Terminal Ready control output (FPGA input)
EXT_UART_RX C12 3.3V UART receive
EXT_UART_TX C11 3.3V UART transmit

Temperature sensor

Lime-GPSDO has integrated temperature sensor which can be used to monitor board temperature through I2C interface.
Sensor has overtemperature shutdown output connected to FPGA. Which can be used to take actions to reduce board temperature when it rises below set limits. For example, fan will be turned on if board will heat up to 55°C and FAN will be turned off if board will cool down to 45°C.

Figure 7 FAN control temperature hysteresis
Table 7. Temperature sensor pins
Schematic signal name FPGA pin I/O standard Comment
FPGA_I2C_SCL J1 3.3V Serial Clock
FPGA_I2C_SDA K1 3.3V Data
LM75_OS H1 3.3V Overtemperature shutdown output (FPGA input)

Clock Distribution

Lime-GPSDO board has onboard 30.72 MHz VCOCXO that is reference clock output. Clock distribution block diagram is presented in Figure 8.

Figure 8 Lime-GPSDO board clock distribution block diagram

VCOCXO frequency can be tuned by DAC (IC2). There is voltage control input of VCOCXO exposed and connected to J2 pin header. By removing R10 resistor and providing control voltage trough J2 header VCOCXO frequency can be tuned externally. Refer to XO1 Rakon U7475LF datasheet for valid control voltage ranges.

J6 SMA connector source can be changed from IC1 clock buffer (default configuration) to FPGA (IC6) clock output by removing R12 and fitting R14 resistors. Refer to Figure 9.

Figure 9 SMA connector J6 source selection