Implementation of a DHCP client using AVR and W5100

In this article, we will take a look at an implementation of DHCP using Wiznet’s W5100 chip. The code is available for download. The TCP/IP stack is hardwired in the W5100 chip. We will only implement an incomplete version of DHCP. It will make the code relatively short so that it won’t be heavy on the microcontroller’s memory. It also keeps the code simpler and easier to understand. It is sufficient for most hobbyist and student projects.

Wiznet provides a library for implementing DHCP.  Some code has been taken from the library for our purpose. The code runs upon another library (called the driver) provided by Wiznet. It provides a BSD standard access to the W5100’s TCP/IP stack. The driver first needs to be configured for your particular microcontroller. It is assumed that you have already done that.

Detailed information about DHCP (Dynamic Host Configuration Protocol) is available in RFC 2131 and RFC 2132.

The code has been written for Atmel’s ATmega16 AVR microcontroller. The functionality has been implemented in a single function which is meant to be called only once during the program. It returns 1 on success and 0 on failure.

The working of the program is explained in the following flowchart:

Flowchart for incomplete DHCP client program

Flowchart for the DHCP client program

The code is available for download in two files. The file dhcp.h contains macro definitions and declarations. The file dhcp.c contains the definition of our function.

Click here to download the code

Graphical analog clock written in Assembly

A screenshot of the clock running on DOSBox.

A screenshot of the clock running on DOSBox.

While I took a course about microprocessor architecture at college, I worked on a program that showed time in analog and digital format. The code was written in assembly language (MASM in particular). I did not write the code from scratch. The original code is available from multiple Chinese websites. I have not been able to determine the original writer.

The original code consisted of a customizable analog clock. It used basic DOS interrupts to draw the graphics. Its size and colour could be changed during runtime. It also had some sound functionality. The code was written for Intel’s 80286 processor and a compatible version of DOS. The program ran fine as long as one did not play with the its customizable features.

I removed the buggy features from the code and added some new (non-buggy) ones. I stripped the code of runtime customizations and sound. And I added a needle for milliseconds, along with a digital clock. I used MASM 6.15 to compile the code. The program can be run in Windows XP. For newer versions of windows, a virtual machine is not able to run it very smoothly. A better option is DOSBox. Be sure to set cycles=max in the configuration file (options).

Following is a simple flowchart for the code.

Flowchart for the clock's assembly code.

Flowchart for the clock’s assembly code.

Download the source: masm-clock at GitHub

Communicating with W5100 via SPI

The purpose of this article is to explain the process of communication between a microcontroller and Wiznet’s W5100 networking chip. Of the three methods of interfacing with W5100, Serial Peripheral Interface (SPI) is the method of choice at least for all non-commercial work. Basic knowledge of SPI is assumed. The code is written for AVR ATmega16.

The first thing to do is to initialize the SPI.


#define DDR_SS_PORT     DDRD
#define SS_PORT         PORTD
#define SS_PIN          PD2
#define DDR_SPI_PORT    DDRB
#define SPI_PORT        PORTB
#define MISO_PIN        PB6
#define MOSI_PIN        PB5
#define SCK_PIN         PB7

void SpiInit(void)
{
    DDR_SS_PORT |= (1<<SS_PIN);                   // SS: Slave Select
    DDR_SPI_PORT |= (1<<MOSI_PIN) | (1<<SCK_PIN); // MOSI and SCK

    SPI_PORT |= (1<<MISO_PIN);       // Enabling pull-up register
    SS_PORT |= (1<<SS_PIN);          // Disabling slave

    // Enable, Master. Xtal/4. MSB first. Mode 0.
    SPCR |= (1<<SPE) | (1<<MSTR); } 

Now we come to the real part. Communication with W5100 occurs in 32-bit streams. Each stream takes 4 SPI cycles, one for each byte. The first byte is the opcode. There are only two operations: READ and WRITE. The second and third bytes are the address to the memory position in W5100 which is to be accessed. In case of a WRITE operation, the last byte indicates the value to be written. In case of a READ operation, the last byte has no purpose except to push the desired data out of W5100. The following function simply reads the value at the 16-bit address given in the parameter.

 uint8_t readW5100(uint16_t addr) { 	// Divide address into two bytes 	uint8_t addr0 = (uint8_t)(addr&0x0F); 	uint8_t addr1 = (uint8_t)((addr&0xF0)>>8);

	SS_PORT &= ~(1<<SS_PIN);      // Select slave

	SPDR = 0x0F;                  // 'Read' operation
	while(!(SPSR & (1<<SPIF)));

	SPDR = addr1;                 // Send address
	while(!(SPSR & (1<<SPIF)));
	SPDR = addr0;
	while(!(SPSR & (1<<SPIF)));

	SPDR = 0xFF;                  // Read data
	while(!(SPSR & (1<<SPIF)));

	SS_PORT |= (1<<SS_PIN);       // De-select slave 	return SPDR; } 

A similar function is used for writing the W5100 memory:

 void writeW5100(uint16_t addr, char data) { 	uint8_t addr0 = (uint8_t)(addr&0x0F); 	uint8_t addr1 = (uint8_t)((addr&0xF0)>>8);

	PORTD &= ~(1<<SS);          // Select slave

	SPDR = 0xF0;                // 'Write' operation
	while(!(SPSR & (1<<SPIF)));

	SPDR = addr1;               // Send address
	while(!(SPSR & (1<<SPIF)));
	SPDR = addr0;
	while(!(SPSR & (1<<SPIF)));

	SPDR = data;                // Send data
	while(!(SPSR & (1<<SPIF)));

	PORTD |= (1<<SS);
}

The motivation for this article comes from the experience that many of the libraries available for W5100 socket programming, including the one provided by Wiznet, do not contain definitions of the elementary functions that are discussed here. The reason given for this is the platform-dependent nature of the functions.

The code given in this post can be helpful for running W5100 from scratch without the use of any library. It can also be helpful for adapting a library to your system in order to make it functional.