This is ENC28J60 driver practically platform agnostic, yet targetting mainly baremetal systems. Supports asynchronous writting and reading from enc28j60's internal buffer (fails back to synchronous transfers if the platform does not support asynchronous SPI IO), and is compliant with errata sheet for all versions. Comes with supplied examples and Doxygen documentation. Licensed under MIT license. To learn how to correctly use this library to comply with the errata sheet, see below.

How to correctly use this library

When working with enc28j60, keep in mind following tips in order for your code to comply with errata sheet:

Make sure speed of your SPI is at least 8 MHz, or
use the same 25 MHz oscillator and generate spi clock of 25/2, 25/3, 25/4 ... MHz (errata #1; for B1 and B4 only)
When using enc28j60_init_rxbuffer(), make sure that the RX buffer starts at 0000h (errata #5)
Use enc28j60_readrxframescount() instead of checking for PKTIF flag (errata #6)
Once the transmission finishes, make sure to call enc28j60_aftertx(), enc28j60_readtxstatus() and enc28j60_shouldretransmit() with additional code to retransmit the frame in case of late collision (errata #12, #14, #15; also note that LATECOL flag is unreliable, this is why it's important to call enc28j60_readtxstatus() and check for late collision there using txstatus.status & ENC28J60_TXSTATUS_LATE_COLLISION - which is done inside enc28j60_shouldretransmit()):
unsigned remaining_retries = 16;
enc28j60_txstatus status;
do {
enc28j60_cleartxflags(&eth1);
enc28j60_transmitframe(&eth1);
enc28j60_jointx(&eth1); // or wait for interrupt
enc28j60_aftertx(&eth1);
enc28j60_readtxstatus(&eth1, tx_start, tx_length, &status);
enc28j60_decrementretries(&remaining_retries);
} while(enc28j60_shouldretransmit(&status, remaining_retries));
When sending frames, do CRC calculation in software, as doing this by enc28j60 will abort currently received packet. (errata #18)
When using pattern match filters, note that extra packets not fitting the filter may be received. (errata #18)
Note that enc28j60 does not support auto negotiation. That is, with most equipment, it can communicate in only half-duplex unless both ends are manually configured to work in full-duplex (Section 9.0 of datasheet)

Errata

Module	Feature	#	Summary	B1	B4	B5	B7	Implemented in/by
MAC Interface	-	1.	MAC registers unreliable with slow asynchronous SPI clock	X	X			user by setting correct SPI speed
Reset	-	2.	CLKRDY set early	X	X	X	X	`enc28j60_softreset()`
Core	Operating Specifications	3.	Industrial (-40°C to +85°C) temperature range unsupported	X	X			-
Oscillator	CLKOUT pin	4.	CLKOUT unavailable in Power Save mode	X	X	X	X	-
Memory	Ethernet Buffer	5.	Receive buffer must start at 0000h	X	X	X	X	user when using `enc28j60_init_rxbuffer()`
Interrupts	-	6.	Receive Packet Pending Interrupt Flag (PKTIF) unreliable	X	X	X	X	user by using `enc28j60_readrxframescount()` instead of checking for `PKTIF`
PHY	-	7.	TPIN+/- automatic polarity detection and correction unreliable	X	X	X	X	user by avoiding incorrect RX polarity
PHY	-	8.	RBIAS resistor value differs between silicon revisions	X	X			user when designing board
PHY	-	9.	Internal loopback in half-duplex unreliable	X	X	X	X	`enc28j60_init()`
PHY	-	10.	Internal loopback in full-duplex unreliable	X	X	X	X	user
PHY LEDs	-	11.	Combined Collision and Duplex Status mode unavailable	X	X	X	X	user
Transmit logic	-	12.	Transmit abort may stall transmit logic	X	X	X	X	`enc28j60_aftertx()`
PHY	-	13.	Received link pulses potentially cause collisions			X	X	`enc28j60_shouldretransmit()`
Memory	Ethernet Buffer	14.	Even values in ERXRDPT may corrupt receive buffer	X	X	X	X	`enc28j60_advancerxreadpointer()` or `enc28j60_finishreadframe()`
Transmit logic	-	15.	LATECOL Status bit unreliable	X	X	X	X	`enc28j60_readtxstatus()`
PHY LEDs	-	16.	LED auto-polarity detection unreliable	X	X	X	X	user
DMA	-	17.	DMA checksum calculations will abort receive packets	X	X	X	X	user by doing checksum in software
Receive Filter	-	18.	Pattern match filter allows reception of extra packets	X	X	X	X	user by discarding unwanted packets
SPI	-	19.	Reset command unavailable in Power Save mode	X	X	X	X	`enc28j60_softreset()`

Example usage

First copy enc28j60-struct.example.h into enc28j60-struct.h and make changes into the code, structures and macros if you wish. You have to choose whenever your system supports advanced spi driver (that is a driver that can transceive to/from two different buffers while keeping SS low all the time). In this guide, we assume that you have advanced driver, but you can find code examples even for the basic driver in /examples/*_nonadv.c.

After customizing the enc28j60-struct.h, implement the functions defined there (enc28j60_spi_transfer() if you don't have advanced SPI driver, enc28j60_spi_advanced_transfer() otherwise; enc28j60_spi_transfer_join(), enc28j60_spi_error_handler(), enc28j60_delay() and enc28j60_getticks()).

Initialization

Choose whenever to use full duplex (if available - otherwise half-duplex will be used even if is_full_duplex is true) and whenever to discard frames with invalid CRC and finally chose MAC address. Reset the device and initialize it with chosen parameters.

enc28j60 eth0;
/* Reset and initialize enc28j60 */
enc28j60_init_struct init_struct = {
    .is_full_duplex = true,
    .discard_crc_errors = true
};
char mac[] = { 0x00, 0x00, 0x00, 0x01, 0x02, 0x03};
memcpy(init_struct.mac_address, mac, 6);
printf("Initializing\n");
enc28j60_softreset(&eth0);
enc28j60_init(&eth0, &init_struct);
...

Note that you can modify the struct enc28j60 and add for example a pointer to the SPI device you wish to use. Then the first lines will look like this:

enc28j60 eth0;
eth0.spi_pointer = &spi1_dma;
/* Reset and initialize enc28j60 */
... 

Reception

It's possible to setup interrupts to handle reception of frames, however to keep things simple we start with simple blocking architecture. First you can choose, whenever you wish to accept broadcast and multicast packets, set where RX buffer starts and ends, and then finally enable the receiver and wait for frames.

...
enc28j60_enablebroadcast(&eth0, true);
enc28j60_enablemulticast(&eth0, true);
// use whole memory as receive buffer
enc28j60_init_rxbuffer(&eth0, 0, ENC28J60_MEMORY_SIZE-1);
/* Enable receiver and start receiving packets */
printf("Receiving...\n");
enc28j60_enablerx(&eth0);
static char frame[ENC28J60_MAX_FRAMELEN_ETH + sizeof(enc28j60_header)];
while(1) {
    uint8_t cnt;
    do {
        // Loop until we have received any frame
        enc28j60_readrxframescount(&eth0, &cnt);
    } while(cnt == 0);
    // or call the following code in interrupt (EIR.PKTIF - it is reliable as source of interrupt, but unreliable when you read it from EIR register)
    enc28j60_readframe(&eth0, frame, sizeof(frame), NULL);
    enc28j60_finishreadframe(&eth0); 
    unsigned length = eth0.length;
    do_whatever_you_want_with_frame(frame, length);
}

or, if you want to prototype faster, you can just use built-in function enc28j60_receiveframeblocking() which either waits at most given number of ticks, or waits infinitely if 0 is supplied as timeout argument until a frame is received (if you don't want this function to block, simply check the number of received frames yourself before calling this function):

...
while(1) {
    enc28j60_receiveframeblocking(&eth0, frame, sizeof(frame), 0);
    unsigned length = eth0.length;
    do_whatever_you_want_with_frame(frame, length);
}

But note that this function blocks - if you don't want it to block, you can simply check the amount of received frames, and if at least one was received, the call to enc28j60_receciveframeblocking() won't wait, but immediately start reading that frame from enc28j60's memory.

Transmission

The transmission is simple as well, and the code below can be divided into two pieces, one part of which gets executed somewhere in the application, while the other part might get executed from interrupt.

...
char tx_frame[] = {...}; // ethernet frame with valid CRC at the end
uint16_t start_ptr = ENC28J60_MEMORY_SIZE - 0x600;
enc28j60_writeframe(&eth0, 0,
    start_ptr, tx_frame, sizeof(tx_frame), NULL
);
enc28j60_init_txbuffer(&eth0, start_ptr, sizeof(tx_frame));
printf("Transmitting...\t");
unsigned remaining_retries = 16;
enc28j60_txstatus status;
do {
    enc28j60_cleartxflags(&eth0);
    enc28j60_transmitframe(&eth0);
    enc28j60_jointx(&eth0); 
    // enc28j60_jointx() or run the following code from interrupt on EIR.TXIF or EIR.TXERIF flag
    enc28j60_aftertx(&eth0);
    enc28j60_readtxstatus(&eth0, start_ptr, sizeof(tx_frame), &status);
    enc28j60_decrementretries(&remaining_retries);
} while(enc28j60_shouldretransmit(&status, remaining_retries));
printf("done\n");

This code may seem non-trivial, but without the extra effort it would not be compliant with errata #12, #13, #15, and the transceiver could freeze. The last few lines can be however simplified by using built-in function enc28j60_transmitframeblocking():

printf("Transmitting...\t");
enc28j60_transmitframeblocking(&eth0, start_ptr, sizeof(tx_frame));
printf("done\n");