Lime-GPSDO v1.0 hardware description

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.



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.





Board components description listed in the Table 1.

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.

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.

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.



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.

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.

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.

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.



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.



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.

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.

Power Distribution
Lime-GPSDO board can be powered from USB port. In applications where USB power is insufficient board can be powered from external 5V power supply. External power supply can be fed to J14 barrel power connector by using power plug (1.35mm ID, 3.5mm OD) or pin header J13 (GND and VCC). Also there is an option to power up board from J8 GNSS USB pin header. External power supply connections has automatic source selection between USB and external source with polarity protection. Lime-GPSDO board power distribution block diagram is presented in Figure 10.