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LimeNET-Micro v2.1 hardware description

1 LimeNET-Micro Board Key Features

LimeNET Micro makes deploying wireless networks more accessible than ever before, by extending the LimeNET line of integrated hardware solutions via an ultra-low cost platform that is capable of supporting narrowband systems, such as GSM and IoT wireless standards, in a stand-alone configuration.

Figure 1. LimeSDR-Micro v2.1

LimeNET-Micro board features:

  • RF transceiver
    • Lime Microsystems LMS7002M
  • Raspberry Pi
    • DDR2 SODIMM socket for raspberry Pi CM3(L)
    • 2x downstream USB 2.0 ports
    • Upstream Micro-USB for USB boot of Compute Module
    • 3.5mm analog audio, composite video output
    • Ethernet + active PoE (RJ45) connector
    • Ribbon cable camera and display connectors
    • HDMI connector
    • External UART connector
    • uSD card slot
    • USB2.0 HUB and 10/100 Ethernet controller
    • SDIO Wi-Fi module
    • Jumper for Raspberry boot mode selection
  • FPGA Features
    • Intel 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
  • USB Controller
    • FTDI FT601 USB to FIFO interface bridge chip (connects FPGA to USB2.0 HUB)
  • Memory Devices
    • 4Mbit FLASH (for FPGA data)
    • 2x 128Kbit (16K x 8) EEPROM (for LMS MCU firmware and FPGA data)
  • Other Devices
    • Temperature sensor
    • Crypto Authentication Device
    • GNSS receiver
  • Connections
    • 2 x coaxial RF (SMA) connectors
    • SMA connectors for reference clock IN/OUT, GNSS and Wi-Fi antennas
    • 5V DC power jack
    • 5V header for powering external devices
    • Fan header
    • FPGA JTAG connectors (0.05” pitch and side connector)
    • Backup battery connector for GNSS receiver
  • 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
  • User I/O
    • GPIO header (0.05” pitch) connected to FPGA
    • 5X dual color (RG) LEDs connected to FPGA
    • Push button connected to FPGA
    • Buzzer connected to FPGA
  • Board Size: without connectors 125mm x 65mm (4.92” x 2.56”)

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

  • FT601 controller resources [link]
  • MAX10 device family, refer to Intel documentation [link]
  • LMS7002M transceiver resources [link]

2 Board Overview

LimeNET-Micro board version 2.1 picture with highlighted major components and connections presented in Figure 2 and Figure 3. Lime Microsystems LMS7002M RF transceiver and Intel MAX10 FPGA can be found on bottom side of board, DDR2 SODIMM socket for Raspberry PI Compute module is placed on top side. There are three connector types – data and debugging (USB2.0, FPGA GPIO, Ethernet and JTAG), power (DC jack and external supply pinheader), RF (clock source output, GNSS antenna and time pulse output, RF transmit and receive ports). LimeNET-Micro 2.1 board comes in three versions and each version differs in components which are fitted on board (BOM – Bill Of Material). BOM version can be determined from resistors R34, R35, R36, R37 fitted on board (see Figure 3 “BOM version resistors” and Table 1 for version description).

Table 1. BOM version description
BOM_VER R34 R35 R36 R37 Comment
0 Fitted Fitted Fitted Fitted Lime (full version)
1 NF Fitted Fitted Fitted SDR (without duplexers and WiFi module)
2 Fitted NF Fitted Fitted GSM (with duplexers, without PoE, audio and video)

Figure 2. LimeNET-Micro top side components and connectors

Figure 3. LimeNET-Micro bottom side components

Board components description listed in the Table 2 and Table 3.

Table 2. Board components
Featured devices
Board Reference Type Description
IC1 RF transceiver Lime Microsystems LMS7002M
IC14 GNSS module GNSS receiver
IC12 FPGA Intel MAX10, 10M16SAU169C8G, 169-UBGA
IC24 USB 2.0 HUB Microchip Technology LAN9514 USB 2.0 Hub and 10/100 Ethernet Controller
IC27 USB to FIFO FTDI FT601, USB to FIFO interface bridge
IC30 Wi-Fi SDIO Wi-Fi module (WF111)
Miscellaneous devices onboard
IC13 IC Temperature sensor LM75
IC20/21 IC I2C secure key storage ATECC508A
Configuration, Status and Setup Elements
J3 JTAG chain pin header FPGA programming pin header for Altera USB-Blaster download cable, side connection.
J4 JTAG chain pin header FPGA programming pin header for Altera USB-Blaster download cable, 0.05” pitch


Red-green status LEDs User defined FPGA indication.
LED6 Red-green status LEDs VCC3P3 power rail indications
LS1 Buzzer Buzzer (WST-1203UX), connected to FPGA.
J14 Jumper USB boot mode jumper
J18 Jumper External USB mode jumper
J27 Jumper Power source selection
General User Input/Output
J5 Pin header 8 FPGA GPIOs plus 2 power pins, 0.05” pitch
SB1 Push button Push button connected to FPGA
Memory Devices
IC3, IC16 EEPROM 128K (16K x 8) EEPROM connected to FPGA and LMS7002M
IC23 Flash memory 4Mbit FLASH memory connected to FPGA
Communication Ports and interfaces
J9 RJ45 connector Ethernet + active PoE connector
J10 USB2.0 connector 2x downstream USB2.0 type A connectors
J11 Header USB downstream header
J12 Header Upstream USB2.0 header
J13 Socket DDR2 SODIMM socket for Raspberry Pi CM3(L)
J15 USB2.0 connector microUSB2.0 upstream connector
J17 Header External UART header
J19 Connector HDMI connector for display
J22 Connector LVDS connector for display
J23 Connector LVDS connector for camera
Table 3. Board components
Clock Circuitry
Board Reference Type Description
XO1 VCOCXO 30.72 MHz voltage-controlled crystal oscillator used as a reference clock.
XO2 VCTCXO 30.72 MHz voltage-controlled, temperature compensated crystal oscillator used as a reference clock.
XO3 VCTCXO 30.72 MHz voltage-controlled, temperature compensated crystal oscillator used as a reference clock. (Not fitted by default)
XO4 VCTCXO 40.0 MHz voltage-controlled, temperature compensated crystal oscillator used as a reference clock.
IC35 OR IC37 IC Analog devices 16bit AD5662 or 8bit AD5601 Digital-to-analog converter for reference clock voltage control
IC36 IC Phase detector (ADF4002)
J25 SMA connector Reference clock input
J24 SMA connector Reference clock output
RF connectors
J1 SMA connector Transmit port or TRXA channel in duplex mode
J2 SMA connector Receive port or TRXB channel in duplex mode
J7 SMA connector GNSS module antenna connector
J20 SMA connector Antenna connector for Wi-Fi module
Power Supply
J28 DC input jack External 5V DC power supply input
J26 Pin header 5V DC power supply output header and main internal power rail
J6 Pin header Backup battery connection header for GNSS module. Typical 3.0V (follow recommendation in fitted GNSS module datasheet)
J16 Card slot uSD card slot for Raspberry Pi boot
J11 Header Fan connection header

3 LimeNET-Micro board architecture

Board periphery can be divided in two groups – FPGA controlled periphery and Raspberry controlled periphery. Intel MAX10 FPGA acts as a glue logic between onboard periphery such as LMS7002M RF transceiver, FT601 USB-to-FIFO IC and Raspberry CM3. Due to FPGA reprogrammable nature additional logic can be implemented also. Raspberry CM3(L) can be used to implement various applications. The block diagram for LimeNET-Micro board is presented in Figure 4.

Figure 4. LimeNET-Micro Development Board Block Diagram

3.1 RF transceiver

The LMS7002M is a fully-integrated, multi-band, multi-standard RF transceiver that is highly programmable. It combines Low Noise Amplifiers (RXLNA), TX Power Amplifier Drivers (TXPAD) receiver/transmitter (RX/TX) mixers, RX/TX filters, synthesizers, RX gain control, TX power control, analogue-to-digital and digital-to-analogue converters (ADC/DACs) and has been designed to require very few external components.

For RF path description connected to Lime Microsystems RF transceiver LMS7002M refer to Figure 5. There are two DPDT switches which selects between LMS7002M RF ports and matching networks. Optionally there can be duplexers fitted onboard and path with duplexers or without them are selected with SPDT switches.

Figure 5. LMS7002M RF path

LMS7002 is connected to Intel MAX10 FPGA. The interface and control signals are described below:

  • Digital Interface Signals: LMS7002M is using data bus LMS_DIQ1_D[11:0] and LMS_DIQ2_D[11:0], LMS_ENABLE_IQSEL1 and LMS_ENABLE_IQSEL2, LMS_FCLK1 and LMS_FCLK2, LMS_MCLK1 and LMS_MCLK2 signals to transfer data to/from FPGA. Indexes 1 and 2 indicate transceiver digital data PORT-1 or PORT-2. Any of these ports can be used to transmit or receive data. By default, PORT-1 is selected as transmit port and PORT-2 is selected as receiver port. The FCLK# is input clock and MCLK# is output clock for LMS7002M transceiver. TXNRX signals sets ports directions. For LMS7002M interface timing details refer to LMS7002M transceiver datasheet page 12-13. [link]
  • LMS Control Signals: these signals are used for optional functionality:
    • LMS_RXEN, LMS_TXEN – receiver and transmitter enable/disable signals
    • LMS_RESET – LMS7002M reset;
  • SPI Interface: LMS7002M transceiver is configured via 4-wire SPI interface; FPGA_SPI0_SCLK, FPGA_SPI0_MOSI, FPGA_SPI0_MISO, FPGA_SPI0_LMS_SS. The SPI interface controlled from FPGA.
  • LMS I2C Interface: Connected to LMS EEPROM.

For connection details see Table 4.

Table 4. RF transceiver (LMS7002) digital interface pins
Chip pin (IC1) Chip reference (IC1) Schematic signal name FPGA pin FPGA I/O standard Description
AM24 xoscin_rxx RxPLL_CLK NC 3.3V Connected to 30.72 MHz clock
V34 RXEN LMS_RXEN - - Connected to shift register
H30 DIQ2_D0 LMS_DIQ2_D0 M1 3.3V
J31 DIQ2_D1 LMS_DIQ2_D1 L5 3.3V
K30 DIQ2_D2 LMS_DIQ2_D2 K2 3.3V
K32 DIQ2_D3 LMS_DIQ2_D3 M2 3.3V
L31 DIQ2_D4 LMS_DIQ2_D4 M4 3.3V
K34 DIQ2_D5 LMS_DIQ2_D5 J1 3.3V
M30 DIQ2_D6 LMS_DIQ2_D6 L2 3.3V
M32 DIQ2_D7 LMS_DIQ2_D7 K1 3.3V
N31 DIQ2_D8 LMS_DIQ2_D8 N4 3.3V
N33 DIQ2_D9 LMS_DIQ2_D9 J2 3.3V
P30 DIQ2_D10 LMS_DIQ2_D10 L4 3.3V
P32 DIQ2_D11 LMS_DIQ2_D11 N2 3.3V
E5 xoscin_txx TxPLL_CLK NC 3.3V Connected to 30.72 MHz clock
U29 TXEN LMS_TXEN - - Connected to shift register
AG31 DIQ1_D0 LMS_DIQ1_D0 N12 3.3V
AF30 DIQ1_D1 LMS_DIQ1_D1 M12 3.3V
AF34 DIQ1_D2 LMS_DIQ1_D2 N11 3.3V
AE31 DIQ1_D3 LMS_DIQ1_D3 L10 3.3V
AD30 DIQ1_D4 LMS_DIQ1_D4 N9 3.3V
AC29 DIQ1_D5 LMS_DIQ1_D5 M13 3.3V
AE33 DIQ1_D6 LMS_DIQ1_D6 M10 3.3V
AD32 DIQ1_D7 LMS_DIQ1_D7 N10 3.3V
AC31 DIQ1_D8 LMS_DIQ1_D8 M8 3.3V
AC33 DIQ1_D9 LMS_DIQ1_D9 M9 3.3V
AB30 DIQ1_D10 LMS_DIQ1_D10 K6 3.3V
AB32 DIQ1_D11 LMS_DIQ1_D11 K7 3.3V
U33 CORE_LDO_x LMS_CORE_LDO_EN - - Connected to shift register
E27 RESET LMS_RESET - - Connected to shift register
D28 SEN FPGA_SPI_LMS_SS J5 3.3V SPI interface
C29 SCLK FPGA_SPI_SCLK M5 3.3V SPI interface
F30 SDIO FPGA_SPI_MOSI K5 3.3V SPI interface
F28 SDO FPGA_SPI_MISO N5 3.3V SPI interface
D26 SDA LMS_I2C_SDA - - Connected to EEPROM
C27 SCL LMS_I2C_SCL - - Connected to EEPROM

3.2 Raspberry Pi Compute Module 3

LimeNET-Micro board has DDR2 SODIMM socket which is dedicated only for Raspberry Pi Compute Module 3 (CM3 OR CM3L) connection. CM3 module has various periphery and FPGA connected to it. For detailed connection between CM3 and FPGA see Table 5.

Table 5. Raspberry Pi Compute Module 3 and FPGA connection.
Conn. pin Schematic signal name FPGA pin I/O standard
35 RAPI_SPI0_SCLK F13 3.3V
33 RAPI_SPI0_MOSI M11 3.3V
27 RAPI_SPI0_CE0 E13 3.3V
71 RAPI_SPI1_SCLK G10 3.3V
69 RAPI_SPI1_MOSI K13 3.3V
65 RAPI_SPI1_MISO L11 3.3V
63 RAPI_SPI1_CE0 A10 3.3V
76 RAPI_GPCLK1 H5 3.3V
45 RAPI_GPIO12 F10 3.3V

3.3 USB subsystem

LimeNet-Micro has a versatile internal USB subsystem. USB subsystem main components are Raspberry CM, switches array, configuration jumpers and connectors. USB subsystem is presented in the picture below.

Figure 6. LimeNet-Micro internal USB subsystem

Internal USB hub (IC24) has five USB ports.

Table 6. Internal USB hub (IC24) ports
Conn. pin Port type Connections Comment
USB0 upstream Connected to switches array and to header (J12) for debug purposes.
USB1 downstream Internal connection to Ethernet controller
USB2 downstream Bottom USB-A port (J10) Can be used to connect various peripherals.

These ports (J10) is powered from same power rail and current is limited (default 0.6A total).

Current limit for these ports can be changed or bypassed manually.

USB3 downstream Top USB-A port (J10)
USB4 downstream Connected to header (J11) and via 0R resistors to GNSS (IC14) module
USB5 downstream Directly connected to USB FIFO interface FT601 (IC27).

There are three USB subsystem modes:

  1. Normal mode. Raspberry compute module USB downstream via switches is connected to internal USB hub/Ethernet controller (IC24). For this mode jumper J14 and J18 must not be mounted. Micro USB (J15) can be used only to power board depending on power jumper J27 position.
  2. External USB mode. Internal USB hub upstream is connected to Micro USB (J15) connector and can be connected to external host. In this case all LimeNet-Micro USB devices are visible to external system. For this mode jumper J14 must be not mounted and J18 must be mounted.
  3. External USB boot. Raspberry Compute module internal memory eMMC or uSD card (depending on CM model) can be updated in this mode via micro USB (J15). For this mode jumper J14 must be mounted and J18 must be not mounted. To enter USB boot mode jumpers J14 and J18 positions must be changed and power reapplied. Specific software is required to flash memory. More info [link]

3.3.1 USB-to-FIFO bridge

For data transfer between FPGA and USB LimeNET-Micro board has FTDI FT601 USB-to-FIFO bridge.

The controller signals description showed below:

  • FT_D[31:0] - FTDI 32-bit data interface is connected to FPGA.
  • FT_TXEn, FT_RXFn, FT_SIWUn, FT_WRn, FT_RDn, FT_OEn, FT_BE[3:0] - FTDI interface control signals.
  • FT_CLK - FTDI interface clock. Clock from FTDI is fed to FPGA.

In the table below are listed USB controller (FTDI) pins, schematic signal name, FPGA interconnections and I/O standard.

Table 7. USB-to-FIFO bridge connection details
Chip pin (IC27) Chip reference (IC27) Schematic signal name FPGA pin I/O standard Comment
40 DATA_0 FT_D0 A2 3.3V
41 DATA_1 FT_D1 A3 3.3V
42 DATA_2 FT_D2 A4 3.3V
43 DATA_3 FT_D3 A5 3.3V
44 DATA_4 FT_D4 A6 3.3V
45 DATA_5 FT_D5 A7 3.3V
46 DATA_6 FT_D6 A8 3.3V
47 DATA_7 FT_D7 A9 3.3V
50 DATA_8 FT_D8 A12 3.3V
51 DATA_9 FT_D9 B13 3.3V
52 DATA_10 FT_D10 C13 3.3V
53 DATA_11 FT_D11 B2 3.3V
54 DATA_12 FT_D12 B3 3.3V
55 DATA_13 FT_D13 B4 3.3V
56 DATA_14 FT_D14 B5 3.3V
57 DATA_15 FT_D15 B6 3.3V
60 DATA_16 FT_D16 D9 3.3V
61 DATA_17 FT_D17 E6 3.3V
62 DATA_18 FT_D18 E8 3.3V
63 DATA_19 FT_D19 F9 3.3V
64 DATA_20 FT_D20 E9 3.3V
65 DATA_21 FT_D21 E10 3.3V
66 DATA_22 FT_D22 F8 3.3V
67 DATA_23 FT_D23 B11 3.3V
69 DATA_24 FT_D24 B12 3.3V
70 DATA_25 FT_D25 C11 3.3V
71 DATA_26 FT_D26 C12 3.3V
72 DATA_27 FT_D27 F12 3.3V
73 DATA_28 FT_D28 G13 3.3V
74 DATA_29 FT_D29 H13 3.3V
75 DATA_30 FT_D30 J13 3.3V
76 DATA_31 FT_D31 L13 3.3V
58 CLK FT_CLK G9 3.3V
4 BE_0 FT_BE0 H9 3.3V
5 BE_1 FT_BE1 G12 3.3V
6 BE_2 FT_BE2 H8 3.3V
7 BE_3 FT_BE3 K11 3.3V
8 TXE_N FT_TXEn J12 3.3V
9 RXF_N FT_RXFn K12 3.3V
10 SIWU_N FT_SIWUn - 3.3V 10k pull up
11 WR_N FT_WRn E12 3.3V
12 RD_N FT_RDn D12 3.3V
13 OE_N FT_OEn J9 3.3V
15 RESET_N FT_RESETn - - 10k pull up, connected to shift register
16 WAKEP_N FT_WAKEUPn H10 3.3V 10k pull up

3.3.2 USB hub + Ethernet

Microchip LAN9514 IC connects all LimeNET-Micro USB periphery and in same time works as Ethernet Controller. For connection details see Table 8 and Table 9.

Table 8. USB hub and Ethernet connection
Chip pin (IC24) Chip reference (IC24) Schematic signal name FPGA pin I/O standard Comment
41 AUTOMDIX_EN ETH_AUTOMDIX_EN - - Connected to shift register
12 NRESET ETH_NRESET - - Connected to shift register
Table 9. RJ45 connector
Connector pin Schematic signal name FPGA pin I/O standard Comment
P7 ETH_LED1 - - Connected to shift register
P13 ETH_LED2 - - Connected to shift register

3.4 GNSS module

GNSS module can be used not only for positioning applications but for frequency measuring and tuning applications also. It provides 1PPS time synchronization pulse which is connected to FPGA.

Table 10. GNSS module connection
Chip pin (IC14) Chip reference (IC14) Schematic signal name FPGA pin I/O standard Comment
20 UART_TX/SPI_MISO GNSS_UART_TX D8 3.3V Also connected to CM3
21 UART_RX/SPI_MOSI GNSS_UART_RX L12 3.3V Also connected to CM3

3.5 GPIO

There are eight general purpose input/output pins from FPGA connected to FPGA GPIO header (J5). Schematic names and pin connections can be found in Table 11.

Table 11. GPIO connection
Connector pin Schematic signal name FPGA pin I/O standard Comment
1 FPGA_GPIO0 L1 3.3V
2 FPGA_GPIO1 M7 3.3V
3 FPGA_GPIO2 N8 3.3V
4 FPGA_GPIO3 J6 3.3V
5 FPGA_GPIO4 D11 3.3V
6 FPGA_GPIO5 J7 3.3V
7 FPGA_GPIO6 A11 3.3V
8 FPGA_GPIO7 J8 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 7).

Figure 7. Power rail selection for pin 10 of J5 connector

3.6 Indication LEDs

LimeNET-Micro board comes with six 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 12. GNSS module connection
Board Reference Schematic name Board label FPGA pin Description
LED1 FPGA_LED1_G LED1 - Connected to shift register.
LED2 FPGA_LED2_G LED2 - Connected to shift register.
LED3 FPGA_LED3_G LED3 - Connected to shift register.
LED4 FPGA_LED4_G LED4 - Connected to shift register.
LED5 FPGA_LED5_G LED5 - Connected to shift register.


- Green LED indicates VCC3P3 power rail presence.

Red LED is unused


3.7 Buzzer and push button

There is one push button and buzzer connected to FPGA. Default functions and connection information are listed in the table below.

Table 13. Buzzer and push button connection
Board Reference Schematic name FPGA pin I/O standard Description

3.8 Communication interfaces (I2C, UART, SPI)

LimeNET-Micro board has various options of communication interfaces. There are I2C, UART and SPI interfaces for controlling onboard periphery. For graphical representation see Figure 8 and detailed description can be found in Table 14.

Figure 8. Communication interfaces

UART interface of GNSS receiver is connected to FPGA and GNSS_UART_TX line is connected to RAPI_UART1_RX through 0R resistor R56 (fitted by default). To connect RAPI_UART1_TX to GNSS_UART_RX line 0R resistor R54 has to be fitted (not fitted by default).

FPGA_I2C interface master can be either FPGA or CM3 module. By default, FPGA_I2C is connected to RAPI_I2C1. In order to connect it to RAPI_I2C0 lines 0R resistors R126, R132 has to be fitted and R130, R134 removed.

Table 14. Communication interface pins
Schematic signal name FPGA pin I/O standard Comment
FPGA_I2C_SCL K10 3.3V Serial Clock
FPGA_I2C_SDA B10 3.3V Data
FPGA_SPI_SCLK M5 3.3V Serial Clock (FPGA output)
FPGA_SPI_MOSI K5 3.3V Data, master output
FPGA_SPI_MISO N5 3.3V Data, master input
FPGA_SPI_LMS_SS J5 3.3V Slave select for LMS7002M
FPGA_SPI_FLASH_SS - - Slave select for FLASH memory, connected to shift register.
FPGA_SPI_DAC_SS - - Slave select for XO VC DAC, connected to shift register.
RAPI_SPI0_SCLK F13 3.3V Serial clock
RAPI_SPI0_MOSI M11 3.3V Data, master output
RAPI_SPI0_MISO K8 3.3V Data, master input
RAPI_SPI0_CE0 E13 3.3V Slave selects
RAPI_SPI1_SCLK G10 3.3V Serial clock
RAPI_SPI1_MOSI K13 3.3V Data, master output
RAPI_SPI1_MISO L11 3.3V Data, master input
RAPI_SPI1_CE0 A10 3.3V Slave selects
GNSS_UART_RX L12 3.3V GNSS UART receive (FPGA output)
GNSS_UART_TX D8 3.3V GNSS UART receive (FPGA input)

3.9 Temperature control

LimeNET-Micro 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 45°C and FAN will be turned off if board will cool down to 35°C. Fan (5V) can be connected to J8 fan header (Figure 10).

Figure 9. FAN control temperature hysteresis

Table 15. Temperature sensor and fan pin connection
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)
FAN_CTLR C5 3.3V Fan control output

Figure 10. Fan connection to J8 header

3.10 Shift registers

In order to save FPGA pins LimeNET-Micro has four shift registers connected in series for controlling onboard periphery. By using 3 pins FPGA can control up to 32 outputs. For FPGA pins see Table 16 and for output description of shift register see Table 17.

Table 16. Shift register connection to FPGA
Schematic signal name FPGA pin I/O standard Comment
SR_DIN B7 3.3V Serial data (connected to first shift register IC15)
SR_SCLK B9 3.3V Shift register clock (connected to all four shift registers – IC15, IC17, IC19, IC22)
SR_LATCH D6 3.3V Shift register storage clock (connected to all four shift registers – IC15, IC17, IC19, IC22)
Table 17. Shift register output description
Shift reg. output Schematic signal name Comment
0 FPGA_LED1_R FPGA LED control
10 ETH_LED1 RJ45 Ethernet connector LED control
12 LMS_RESET LMS7002M IC1 control
16 FPGA_SPI_ADF_SS FPGA_SPI slave select control
19 ETH_AUTOMDIX_EN Ethernet IC24 control
21 FT_RESETn USB FIFO IC27 control
23 -

3.11 Clock Distribution

LimeNET-Micro board has onboard VCOCXO and VCTCXO oscillators fitted. There is an option to select 30.72MHz or 40.0MHz oscillator as a reference clock for LMS7002M IC. Clock distribution block diagram is presented in Figure 11.

Figure 11. Lime-GPSDO board clock distribution block diagram

Oscillator frequency can be tuned by DAC (IC35 or IC37). Also, there is an option to lock oscillator frequency to external reference clock by using phase detector (IC36).

Table 18. LimeNET-Micro clock pins
Source Schematic signal name I/O standard FPGA pin Description
Clock buffer (IC34)
RxPLL_CLK 3.3V - Connected to LMS7002M
TxPLL_CLK - Connected to LMS7002M
REF_CLK_OUT 3.3V - Connected to J24 SMA connector
REF_ADF 3.3V - Connected to IC36
RF transceiver (IC1) LMS_MCLK1 3.3V G5
LMS_FCLK2 3.3V R29
GNSS module IC14 GNSS_TPULSE 3.3V J10 1PPS time pulse output
USB FIFO IC27 FT_CLK 3.3V G9 100MHz USB FIFO clock
CM3 J13 RAPI_GPCLK1 3.3V H5 Configurable clock output from CM3

Clock buffer source selection:

  • Clock oscillator - remove R166, R168, R169 resistors and fit one of the R162, R171, R174 resistors (depends on VCOCXO/VCTCXO selection). This is default board configuration;
  • Raspberry PI (RAPI_GPCLK2) - remove R162, R171, R174 resistors and fit R166 resistor.
  • J25 SMA connector (REF_CLK_IN) - remove R162, R171, R174 resistors and fit R168 resistor.
  • USB hub (ETH_CLK24) - remove R162, R171, R174 resistors and fit R169 resistor.

Clock oscillator selection:

  • XO1 (30.72MHz) – remove R166, R168, R169, R172, R173 and fit R162, R163 resistors. This is default board configuration.
  • XO2/XO3 (30.72Mhz) – fit only one of the oscillators because they share same footprint, remove R162, R163, R166, R168, R169, R173 and fit R171, R172 resistors. By default, XO2 is fitted.
  • XO4 (40MHz) - remove R162, R163, R166, R168, R169, R171, R172 and fit R174, R175 resistors.

3.12 Power Distribution

LimeNET-Micro board can be powered in three ways:

  • Power over Ethernet – using RJ45 connector and power sourcing equipment (PSE) such as PoE capable network switch or PoE injector. LimeNET-Micro board works with PSE supplying power in “Mode A/Endspan” as well in “Mode B/Midspan” modes;
  • DC connector - 5V power source connected to J28 power jack should be capable to provide 2A-3A current;
  • microUSB port - 5V power source connected to J15 connector should be capable to provide 2A-3A current;

Power source can be selected fitting jumper in one of the three positions on J27 header:

  • A (pins 5-6) – 5V micro USB;
  • B (pins 3-4) – 5V external DC;
  • C (pins 1-2) – Power over Ethernet;

LimeNET-Micro board power distribution block diagram is presented in Figure 12.

Figure 12. LimeNET-Micro board power distribution block diagram

4 Getting Started with LimeNET-Micro

4.1 Basic setup

To get LimeNET-Micro board up and running:

  • Set “Power selection” jumper J27 to “A” position (pins 5-6, 5V micro USB power selection);
  • Connect mouse and keyboard to J10 USB connectors;
  • Connect monitor cable to J19 HDMI connector;
  • Insert CM3 module into J13 socket (or CM3(L) module into J13 and uSD card into J16 slot).
  • Connect power adapter (5V, 2A-3A) to micro USB connector J15.

4.2 FPGA programming

Intel MAX10 FPGA can be programmed using JTAG adapter such as Intel FPGA USB Download Cable (formerly known as Altera USB-Blaster download cable). LimeNET-Micro board has two different size and pin count JTAG headers (J4 - 10pin 0.05” pitch header and J3 - 7pin 0.1” pitch edge connector). Additional cable adapter is needed to connect download cable to LimeNET-Micro board because Intel FPGA USB Download Cable comes with 10-pin, 0.1” pitch female connector.

Figure 14. FPGA JTAG pinheaders

4.3 Board status

LimeNET-Micro board has six dual color (red/green) LED indicators fitted on top and two green indicators on Ethernet (J9) connector. Each LED except power status LED is controlled from FPGA and their function can be changed. Default LED function and their other information are listed in the table below.

Table 19. Default LEDs functions
Board Reference Schematic name Board label Type Description


CPU activity CM3, USB FIFO and NIOS CPU activity:

Green - CM3 activity; Red - USB and NIOS CPU activity.



Ethernet/Wi-Fi activity Ethernet/Wi-Fi activity:

Green - Ethernet Link Activity; Red - Wi-Fi module activity.



Clock status Blinking indicates presence of TCXO clock.

Color indicates status of FPGA PLLs that are used for LMS digital interface clocking: Green – both PLLs are locked; Red/Green – PLL is not locked.



TCXO control mode No light – TCXO is controlled from DAC

Red – TCXO is controlled from phase detector and is not locked to external reference clock Green – TCXO is controlled from phase detector and is locked to external reference clock. Red and green at same time – TCXO is controlled from DAC, and its value tuned by PPS signal coming from GPS module.



TCXO tune state 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



Power status Green LED indicates VCC3P3 power rail presence. Red LED is unused
J9 ETH_LED1 - Ethernet Speed

Indicator LED

LED is on when the Ethernet operating speed is 100 Mbs.
J9 ETH_LED2 - Ethernet Link

Activity Indicator LED

LED is on when a valid link is detected and blinking indicates activity.
Figure 15. LED indicators of RJ45 (J9) connector