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.

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.

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-GPSDO 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).

Figure 5 Power rail selection for pin 10 of J12 connector

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

Table 8. Lime-GPSDO clock pins

Source	Schematic signal name	I/O standard	FPGA pin	Description
Clock buffer (IC1)	CLK0_OUT	3.3V	H6	30.72 MHz
	CLK1_OUT	3.3V	-	Connected to J5 SMA connector
	CLK2_OUT	3.3V	-	Connected to J6 SMA connector
GNSS module IC5	GNSS_TPULSE	3.3V	A2	1PPS time pulse output
Oscillator (XO2)	CLK50_FPGA	3.3V	H4	50MHz clock source s
FPGA (IC6)	FPGA_TPULSE	3.3V	C2	FPGA 1PPS time pulse output connected to J3
	FPGA_CLK_OUT	3.3V	L3	Clock output from FPGA, can be connected to J6 SMA connectors

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.

Figure 10 Lime-GPSDO board power distribution block diagram