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APPLICATION NOTE
AT11412: UART to Ethernet Gateway with SAM4S
SMART ARM-Based Microcontroller
Introduction
This application note aims at describing the LwIP stack and how to use it to design a
UART to Ethernet Gateway. This application note will elaborate the software
architecture, internal data transmission scheme and memory footprint to help users
make their own Gateway easily according to the real requirement.
Figure 1.
The UART to Ethernet Gateway
Features










Atmel® SAM4S ARM® Cortext®-M4-based MCU
LwIP stack support
DHCP mode support
UDP server support
TCP client/server support
Support 15 clients in TCP server mode
Ethernet data flow control support
TCP client/server with keep-alive detection and auto reconnection
Multiple UART data package mode
High speed data transfer by DMA
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Table of Contents
1. Kits Overview ..................................................................................... 3
1.1
1.2
SAM4S Xplained Pro Evaluation Kit.................................................................. 3
Ethernet1 Xplained Pro ..................................................................................... 3
2. Application Scenarios......................................................................... 4
3. Software Implementation.................................................................... 5
3.1
3.2
3.3
APIs Introduction ............................................................................................... 5
3.1.1
LwIP Stack Overview .......................................................................... 5
3.1.2
Main APIs Introduction ........................................................................ 5
Internal Mechanism of Data Transmission ........................................................ 8
3.2.1
Data Transfer from UART to Ethernet ................................................ 8
3.2.1.1
UART Data Package Format ............................................ 8
3.2.1.2
UART Data Transfer ......................................................... 8
3.2.1.3
Flow Control for UART Port .............................................. 9
3.2.2
Data Transfer from Ethernet to UART............................................... 10
Quick-start Setup ............................................................................................ 11
4. Footprint........................................................................................... 13
5. Conclusion ....................................................................................... 14
Appendix A.
A.1
Additional Information .................................................. 15
LwIP Configuration .......................................................................................... 15
Appendix B.
Revision History ........................................................... 16
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1.
Kits Overview
1.1
SAM4S Xplained Pro Evaluation Kit
The Atmel® SAM4S Xplained Pro evaluation kit is a hardware platform to evaluate the ATSAM4SD32C microcontroller.
Supported by the Atmel Studio integrated development platform (IDE), the kit provides easy access to the features of
the Atmel ATSAM4SD32C and explains how to integrate the device in a custom design. The Xplained Pro MCU series
evaluation kits include an on-board Embedded Debugger, and no external tools are necessary to program or debug the
ATSAM4SD32C. The Xplained Pro extension series evaluation kits offer additional peripherals to extend the features of
the board and ease the development of custom designs. More details about the SAM4S Xplained Pro evaluation kit are
available here: http://www.atmel.com/tools/ATSAM4S-XPRO.aspx.
Figure 1-1. SAM4S Xplained Pro Evaluation Kit
1.2
Ethernet1 Xplained Pro
Ethernet1 Xplained Pro is a basic extension board for the Xplained Pro platform. The Ethernet is controlled via a SPI
interface up to 40MHz for high throughput Ethernet applications. Ethernet1 Xplained Pro connects to any Xplained Pro
standard extension header on any Xplained Pro MCU board. More details about Ethernet1 Xplained Pro extension
board is available here: http://www.atmel.com/tools/ETHERNET1_XPRO.aspx.
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Figure 1-2. Ethernet1 Xplained Pro Extension Board
2.
Application Scenarios
The Gateway can be used in some IoT applications or other scenarios which need both UART and Ethernet.
The Gateway features one TCP server, one TCP client and one UDP server. The TCP server can allow maximum 15
TCP clients to connect at the same time. The UDP server can receive broadcast message and reply the Gateway IP
information to the remote sender if necessary.
In this demonstration, the IP address of the Gateway is allocated by the DHCP and the external clients can’t know the
Gateway IP address when they want to connect to the Gateway. In order to solve this issue, the Gateway features a
UDP server which can tell the external clients in the same network the IP address of the Gateway when it receives a
broadcast message that contains “Atmel-Gateway” at the header of the message. For example, if there is a mobile
phone that connects to the same network as Gateway, the mobile phone can send a broadcast message “AtmelGateway” through UDP protocol and then the Gateway will reply with a message in the form of “IP address, MAC
address, Description”. External clients can extract the IP address information from this message and then can connect
to the Gateway using this IP address. More clients can connect to the Gateway after they get the Gateway IP address.
The Gateway also behaves as a TCP clients and it will try to connect to a pre-defined server every 5mins if the
connection is not established.
There is one application scenario that the Gateway can be used in a ZigBee network. Figure 2-1 illustrates the
application scenario. The Gateway connects the ZigBee coordinator via UART and connects itself to Ethernet through
network cable. The remote clients can send a UDP broadcast message that contains certain information to get the
Gateway IP address and then connect it to the Gateway. After that, the remote clients can control or get information
from the ZigBee network through the Gateway, such as light infomation. Each client can control the light on or off and if
the light status changed, the Gateway will send this information to all the clients. The Gateway can also connect itself to
a remote server to upload data to remote server or receive command from the server, such as changing configuration
parameters or firmware update.
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Figure 2-1. Example of Application Scenario
Remote Clients
ZigBee
Cordinator
SerialNet AT
Command
UART to Ethernet
Gateway
Internet
Remote Server
3.
Software Implementation
3.1
APIs Introduction
3.1.1
LwIP Stack Overview
The Lightweight TCP/IP stack is designed for embedded systems. The focus of the LwIP TCP/IP implementation is to
reduce resource usage while still having a full scale TCP. This makes LwIP suitable for use in embedded systems with
tens of kilobytes of free RAM and room for around 40 kilobytes of code ROM.
LwIP features:












IP (Internet Protocol) including packet forwarding over multiple network interfaces
ICMP (Internet Control Message Protocol) for network maintenance and debugging
IGMP (Internet Group Management Protocol) for multicast traffic management
UDP (User Datagram Protocol) including experimental UDP-lite extensions
TCP (Transmission Control Protocol) with congestion control, RTT estimation and fast recovery/fast retransmit
DNS (Domain Name Server)
Specialized raw API for enhanced performance
Optional Berkeley-alike socket API
DHCP (Dynamic Host Configuration Protocol)
PPP (Point-to-Point Protocol)
PPPoE (Point to Point Protocol over Ethernet)
ARP (Address Resolution Protocol) for Ethernet
For more details about the LwIP, refer to LwIP Wiki: http://lwip.wikia.com/wiki/LwIP_Wiki or the Atmel AT04055: Using
the LwIP Network Stack application note.
3.1.2
Main APIs Introduction
The main APIs used for the Gateway Ethernet part are the LwIP RAW APIs. The Raw API is a non-blocking, eventdriven API designed to be used without an operating system that implements zero-copy send and receive.
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Table 3-1.
TCP RAW APIs used in this Application
API Function
Description
netif_add
Add a network interface to the list of LwIP netifs
tcp_new
Creates a new connection PCB (Protocol Control Block). A PCB is a structure
used to store connection status.
tcp_bind
Binds the pcb to a local IP address and port number
tcp_listen
Commands a pcb to start listening for incoming connections
tcp_accept
Sets the callback function to call when a new connection arrives on a listening
connection
tcp_connect
Connects to a remote TCP host
tcp_write
Queues up data to be sent
tcp_sent
Sets the callback function that should be called when data has successfully been
sent and acknowledged by the remote host
Receiving TCP
data
tcp_recv
Sets the callback function that will be called when new data arrives
tcp_recved
Informs LwIP core that the application has processed the data
Callback argument
tcp_arg
Specify the argument that should be passed callback functions
tcp_close
Closes a TCP connection with a remote host
tcp_err
Sets the callback function to call when a connection is aborted because of an
error
tcp_abort
Aborts a TCP connection
Network interface
Management
TCP connection
setup
Sending TCP data
Closing and
aborting
connections
 Network interface management
In LwIP device drivers for physical network hardware are represented by a network interface structure similar to
that in BSD. To create a new network interface, a new space should be allocated for the struct netif and call
netif_add():
struct netif *netif_add(struct netif *netif, struct ip_addr *ipaddr,
struct ip_addr *netmask, struct ip_addr *gw, void *state,
err_t (* init)(struct netif *netif),
err_t (* input)(struct pbuf *p, struct netif *netif))
In this application, DHCP mode is used and the ip address, netmask and default gateway don’t need to be
specified when calling netif_add function. These parameters will be set automatically when DHCP client gets a
leased address from the DHCP server successfully.
The init parameter specifies a driver-initialization function that should be called once the netif structure has been
prepared by netif_add.
The final parameter input is the function that a driver will call when it has received a new packet. TCP connection
setup functions
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 TCP connection setup

struct tcp_pcb * tcp_new(void)
Creates a new connection control block (PCB). The connection is initially in the "closed" state. If memory is
not available for creating the new PCB, NULL is returned.

err_t tcp_bind(struct tcp_pcb *pcb, struct ip_addr *ipaddr, u16_t port)
Binds the PCB to a local IP address and port number. The IP address can be specified as IP_ADDR_ANY
in order to bind the connection to all local IP addresses. If the IP address is not given (i.e., ipaddr ==
NULL), the IP address of the outgoing network interface is used instead. If the port is specified as zero, the
function selects an available port. The connection must be in the "closed" state.
If another connection is bound to the same port, the function will return ERR_USE, otherwise ERR_OK is
returned.

struct tcp_pcb * tcp_listen (struct tcp_pcb *pcb)
The "pcb" parameter specifies a connection, which must be in the "closed" state and must have been
bound to a local port with the tcp_bind() function. This functions sets up the local port to listen for
incoming connections.
After calling tcp_listen(), tcp_accept()must be called. Until doing so, incoming connections for this
port will be aborted.
tcp_listen() may return NULL if no memory was available for the listening connection.

void tcp_accept(struct tcp_pcb *pcb, err_t (* accept)(void *arg, struct tcp_pcb
*newpcb, err_t err))
Commands a PCB to start listening for incoming connections. tcp_listen()must have been previously
called. When a new connection arrives on the local port, the specified function will be called with the PCB
for the new connection.

err_t tcp_connect(struct tcp_pcb * pcb, struct ip_addr * ipaddr, u16_t port,
err_t (* connected)(void * arg, struct tcp_pcb * tpcb, err_t err));
Sets up the pcb to connect to the remote host and sends the initial SYN segment which opens the
connection. If the connection has not already been bound to a local port, a local port is assigned to it.
The tcp_connect() function returns immediately; it does not wait for the connection to be properly
setup. Instead, it will call the function specified as the fourth argument (the "connected" argument) when
the connection is established. If the connection could not be properly established, either because the other
host refused the connection or because the other host didn't answer, the error handling function will be
called with an the "err" argument set accordingly.
The tcp_connect() function can return ERR_MEM if no memory is available for enqueueing the SYN
segment. If the SYN indeed was enqueued successfully, the tcp_connect() function returns ERR_OK.
 Sending TCP data

err_t tcp_write(struct tcp_pcb *pcb, const void *data, u16_t len, u8_t apiflags)
Enqueues the data pointed to by the argument dataptr. The length of the data is passed as the len
parameter.
The apiflags argument can have either of the following bits:
TCP_WRITE_FLAG_COPY indicates that LwIP should allocate new memory and copy the data into it. If not
specified, no new memory should be allocated and the data should only be referenced by pointer.
TCP_WRITE_FLAG_MORE indicates that the push flag should not be set in the TCP segment.
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The tcp_write() function will fail and return ERR_MEM if the length of the data exceeds the current send
buffer size or if the length of the queue of outgoing segment is larger than the upper limit defined in
lwipopts.h (TCP_SND_QUEUELEN). If the function returns ERR_MEM, the application should wait until some
of the currently enqueued data has been successfully received by the other host and try again.

tcp_sent(struct tcp_pcb *pcb, tcp_sent_fn sent)
Used to specify the function that should be called when TCP data has been successfully delivered to the
remote host.
 Receiving TCP data

void tcp_recv(struct tcp_pcb *pcb, err_t (* recv)(void *arg, struct tcp_pcb
*tpcb, struct pbuf *p, err_t err))
TCP data reception is callback based; an application-specified callback function is called when new data
arrives.
Sets the callback function that will be called when new data arrives. If there are no errors and the callback
function returns ERR_OK, then it is responsible for freeing the pbuf. Otherwise, it must not free the pbuf so
that LwIP core code can store it. If the remote host closes the connection, the callback function will be
called with a NULL pbuf to indicate that fact.

Close connection

tcp_close(struct tcp_pcb *pcb)
Closes the connection. The function may return ERR_MEM if no memory was available for closing the
connection.If the close succeeds, the function returns ERR_OK.
3.2
Internal Mechanism of Data Transmission
The UART to Ethernet Gateway works between two interfaces. The purpose UART to Ethernet Gateway software is to
get all the information at one interface and send it to the other as quickly as possible. Due to the low speed of serial port
and high speed of Ethernet, several features must be considered during development, such as the data flow control. In
this application, data flow control for the Ethernet interface and data buffer schemes are implemented. There will be a
detail description of internal mechanism of data transmission in the below sections.
3.2.1
Data Transfer from UART to Ethernet
3.2.1.1 UART Data Package Format
There are no package header and tail. The data received will be considered as a valid package if no data received in a
time interval. The time interval can be specified according to the real application. It is 10ms in this Gateway Demo.
3.2.1.2 UART Data Transfer
There are two 2048 bytes data buffer for the UART port. These two buffers work as a ping-pong buffer. One buffer can
be used to receive data from UART and the other can be used to transfer data to Ethernet at the same time.
A dynamically allocated buffer link-list is used for the Ethernet part. Normally, the Ethernet interface is faster than the
UART port, so the buffer link-list is rarely used. However, the network congestion may occur sometimes and in this case
the UART data can be stored in the buffer link-list temporarily.
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Figure 3-1. Flowchart of forwarding UART Data to Network
TCP Send
Buffer
TCP/IP
Connection 0
TCP Send
Buffer
TCP/IP
Connection 1
A
A
TCP Send
Buffer
TCP/IP
Connection N
Buffer0
RX
UART
Buffer1
...
...
...
A
Two 2048 Bytes
data buffer
Dynamically allocated buffer list
In this application, the Gateway will forward the UART data to all the TCP/IP clients that connect to the Gateway. The
TCP send buffer is defined by TCP_SND_BUF in the file lwipopts.h.
Figure 3-2. Flowchart of Software
UART Data Received
N
Valid Frame?
Drop frame
Y
Copy data to allocated buffer
TCP is idle?
N
Add data buffer to the link-list
Y
Copy data to TCP Send Buffer
Sending data through TCP
Free allocated buffer
N
All the data in the buffer link-list has been transferred?
Y
TCP in idle state
Note that the data doesn’t need to be copied into the TCP send buffer in order to save RAM. In this case, the data is
referenced by the pointer and the memory behind the data pointer must not change until the data is ACKed by the
remote host.
3.2.1.3 Flow Control for UART Port
Although there is a dynamically allocated buffer list for the UART data to be stored temporarily in case of network
congestion, it’s better to have a data flow control for the UART port because maybe there aren’t sufficient RAM to be
allocated in some low RAM micro controllers.
There are two flow control schemes, one is hardware flow control and the other is software flow control. Using hardware
flow control, two extra pins (clear-to-send (CTS) and request-to-send (RTS)) in addition to TxD and RxD are used to
stop or start communication at both UART sides. For software flow control, start (XON) and stop (XOFF) commands are
sent as characters, as part of data communication. Only two pins are used (RxD and TxD). However, an extra software
layer must be added to UART software drivers to have this feature.
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In this Gateway demo application, there is no flow control implementation for the UART port.
3.2.2
Data Transfer from Ethernet to UART
The network data is much faster than the UART data transmission, so a data flow control for the network data is
mandatory. Fortunately, TCP provides the data flow control scheme. TCP can negotiate packet lengths to be sent from
one Ethernet device to the other. When one side’s windows buffer becomes full, the other side can slow down its data
rate or even stop until the remote side’s window buffer is available.
The DMA is used for the UART port to speed up the data transmission and to reduce the CPU usage.
Figure 3-3. Flowchart of forwarding Network Data to UART
Packets
TX
UART
0
1
…...
N
TCP Window Buffer
TCP/IP
Connection 0
TCP Window Buffer
TCP/IP
Connection 1
DMA
A
...
...
A
TCP/IP
Connection N
TCP Window Buffer
The TCP window buffer is defined by TCP_WND in the file lwipopts.h.
Figure 3-4. Flowchart of Software
Polling Task
Ethernet Task
Check the data buffer pool
Network data packet received
Network data exist?
Copy data to allocated buffer
N
Y
The same TCP info found in the buffer pool?
DMA is free?
Y
N
Y
Start DMA to transfer data through UART
N
Find a free Buffer pool to store the TCP info
Data transfer finished?
Add data to TCP buffer link-list
N
Y
Free allocated buffer
Inform remote side the data has been taken
N
All the data in the buffer link-list has been transferred?
Y
Remove the TCP info from the buffer pool
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In the Ethernet task, when the network data packet is received, the data is copied into an allocated buffer. Then, check
the buffer pool if there are data packets in the buffer pool that have not been transferred to UART for the same TCP
connection. If yes, add the data buffer to the tail of the link-list. Otherwise, find a free buffer for this TCP connection and
add data to the TCP buffer link-list.
In the polling task, if there is network data in the buffer pool and the DMA is not in use, the DMA will be started and data
will be transferred. At the end of data transfer, the allocated buffer will be freed and the Gateway will inform the remote
side the data has been received, so the remote side can transfer proper size of data packet to the Gateway next time
according to the Gateway available window buffer. If all the data in the buffer link-list has been transferred, the TCP info
will be removed from the buffer pool.
3.3
Quick-start Setup
To evaluate the functionality of this Gateway, a TCP/IP packet tool on PC can be used to setup a quickly and simply
testing.


Connect the Ethernet1 Xplained PRO board to the EXT1 header of SAM4S Xplained PRO board
Power the SAM4S Xplained PRO board via the DEBUG USB interface and connect this board to a network
router through the Ethernet1 Xplained PRO board. Make sure the PC and the board is in the same local area
network.
Figure 3-5. Hardware Setup


Compile the source code and download the firmware image to the board via the on-board embedded debugger

Open the serial tool, such as putty or terminal window in Atmel Studio which can be downloaded from Atmel
Gallery

Power up the board and broadcast “Atmel-Gateway” message using UDP protocol in packet sender tool to get
the IP address of the board
Open the TCP/IP packet tool. Packet Sender is taken as the PC tool just for test purpose which can be found
here: http://packetsender.com/.
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Figure 3-6. Get the Board IP Address
The UDP port number is defined in file src/network/updsev.h

Retrieve the IP address from the received UDP data packet and use this IP address (192.168.1.3) to do data
transfer through TCP protocol
Figure 3-7. Data Transfer through TCP Protocol
The TCP port number is defined in file src/network/server.h.
Packet Sender tool always closes the TCP connection after each data packet transferred. For this reason, data
transfer from serial port to Ethernet can’t be tested. Users can write a test code that can keep long TCP
connection between client and server to do a full test of the Gateway functionalities.
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4.
Footprint
Figure 4-1 and Figure 4-2 illustrate the Flash and RAM spaces that each module used in the software of this
demonstration.
Figure 4-1. UART to Ethernet Gateway RAM Footprint [KB]
Other
2.08
Application
0.19
Low level driver
0.14
UART
4.35
LwIP
26.34
PHY
1.78
Table 4-1.
Primary uses of RAM within LwIP [KB]
UDP PCB
TCP PCB
TCP SEG
PBUF POOL
ARP Table
Others
0.09
2.5
0.47
22.5
0.2
0.58
Figure 4-2. UART to Ethernet Gateway Flash Footprint [KB]
Other
8.06
Application
3.01
Low level driver
3.35
UART
0.77
LwIP
24.07
PHY
2.58
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The RAM consumed by LwIP stack depends on how many clients will connect to the Gateway. From the RAM footprint,
the pbuf consumes most of the RAM. The number pbuf pool can be decreased according to the real requirement if there
are fewer clients that connected to the Gateway. In addition, the system must have sufficient RAM reserved for heap
since the dynamically allocated buffer will be used to store the data that will be sent or received. Much RAM will be
needed if there is network congestion while UART sends data continuously or many clients send data to the Gateway
simultaneously.
5.
Conclusion
This document describes the basic software architecture, the main LwIP APIs used in this application, the internal
scheme of data transmission between UART and Ethernet and a short description about data flow control for both side.
This implementation features a small memory footprint TCP/IP to serial communication for a low-end microcontroller
solution. It does not require in-depth knowledge of Ethernet TCP/IP and can be easily modified to meet different real
application scenarios.
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Appendix A.
A.1
Additional Information
LwIP Configuration
Table A-1 lists the LwIP stack configuration in this application, and these configurations can be modified according to
real application in file config/lwipopts.h.
Table A-1.
LwIP Options for UART to Ethernet Gateway Demonstration
Option
Value
Description
LWIP_TCP
1
Turn on TCP
LWIP_UDP
1
Turn on UDP
LWIP_DHCP
1
Enable DHCP module
MEMP_NUM_TCP_PCB
16
The number of simultaneously active
TCP connections
MEMP_NUM_UDP_PCB
3
The number of UDP protocol control
blocks
MEMP_NUM_TCP_PCB_LISTEN
1
The number of listening TCP
connections
MEMP_NUM_TCP_SEG
30
The number of simultaneously queued
TCP segments
MEMP_NUM_PBUF
2
The number of memp struct pbufs
PBUF_POOL_SIZE
15
The number of buffers in the pbuf pool
PBUF_POOL_BUFSIZE
TCP_MSS+40+PBUF_LINK_HLEN+4
The size of each pbuf in the pbuf pool
TCP_MSS
1460
TCP Maximum segment size
TCP_WND
2 * TCP_MSS
The size of a TCP window
TCP_SND_BUF
2 * TCP_MSS
TCP sender buffer space
MEM_LIBC_MALLOC
1
Use malloc/free/realloc provided by Clibrary
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Appendix B.
Revision History
Doc. Rev.
Date
Comments
42429A
03/2015
Initial document release
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