Chapter 7
Synchronous Serial
SPI and I2C interfaces
Checklist
The following tools will be used in this lesson:


MPLAB X, Integrated Development Environment
(v1.8 or later, free)
MPLAB XC16, C compiler (v1.11 or later, free)
The following pieces of documentation will be used during this lesson:

PIC24FJ128GA010 Datasheet –DS39747 (latest rev.)
Make sure they are available and/or installed and ready to use on your computer.
You can download them from Microchip web site at:
http://www.microchip.com/mplabx
And
http://www.microchip.com/xc16
The following hardware tools are recommended:

Explorer 16 Demonstration Board

PIC24FJ128GA010 PIM

24LC00 or similar Serial EEPROM

2 x 4.5K Ohm resistors
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Synchronous Serial Interfaces
 I2 C

SPI
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
SPI Bus
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Asynchronous Serial Interface
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Comparison of Serial Interfaces
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
The SPI module
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
SPIxCON register
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Initializing SPI2

Pin 12 of PORTD is connected to the memory chip select (CS):
#define CSEE
#define TCSEE

_RD12
_TRISD12
// select line for Serial EEPROM
// tris control for CSEE pin
Peripheral initialization:
// 1. init the SPI peripheral
TCSEE = 0;
//
CSEE = 1;
//
SPI2CON1 = SPI_MASTER; //
SPI2STAT = SPI_ENABLE; //

Where:
#define SPI_MASTER
#define SPI_ENABLE

make SSEE pin output
de-select the Serial EEPROM
select mode
enable the peripheral
0x0120 // 8-bit master mode, CKE=1, CKP =0
0x8000 // enable SPI port, clear status
A small function that will be used to transfer data to and from the serial
EEPROM device:
// send one byte of data and receive one back at the same time
int WriteSPI2( int data)
{
SPI2BUF = data;
// write to buffer for TX
while( !SPI2STATbits.SPIRBF);
// wait transfer completion
return SPI2BUF;
// read the received value
} // WriteSPI2
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
The 25LC256 Serial EEPROM
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


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// 25LC256 Serial EEPROM commands
#define SEE_WRSR
1
// write status register
#define SEE_WRITE
2
// write command
#define SEE_READ
3
// read command
#define SEE_WDI
4
// write disable
#define SEE_STAT
5
// read status register
#define SEE_WEN
6
// write enable
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
25LC256 Status Register
// 2. Check the Serial EEPROM status
CSEE = 0;
// select the Serial EEPROM
writeSPI2( SEE_STAT);
// send a READ STATUS COMMAND
i = writeSPI2( 0);
// send/receive
CSEE = 1;
// deselect, terminate command
// 2.1 send a Write Enable command
CSEE = 0;
// select the Serial EEPROM
WriteSPI2( SEE_WEN);
// write enable command
CSEE = 1;
// deselect, terminate command
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Writing to the EEPROM
// send a Write command
CSEE = 0;
//
WriteSPI2( SEE_WRITE);
//
WriteSPI2( addr_MSB);
//
WriteSPI2( addr_LSB);
//
WriteSPI2( data);
//
// continue writing more data…
CSEE = 1;
select the Serial EEPROM
write command
address MSB first
address LSB (word aligned)
send 8-bit of data
// wait until any work in progress is completed
while ( ReadSR() & 0x1);
// check the WIP flag
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Reading from the EEPROM
// perform a read sequence
CSEE = 0;
// select the Serial EEPROM
WriteSPI2( SEE_READ);
// read command
WriteSPI2( addr_MSB);
// address MSB first
WriteSPI2( addr_LSB);
// address LSB (word aligned)
data = WriteSPI2( 0);
// send dummy, read msb
// continue reading a second byte, a third…
CSEE = 1;
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
I2C Interface Examples
A few examples of devices using the I2C interface :
 1Mbit Serial EEPROMS: 24xx1025
 18-bit delta sigma ADCs: MCP3421
 16-bit delta sigma ADCs: MCP3425
 12-bit SAR ADCs:MCP3221
 12-bit D/A: MCP4725
 Integrated Temperature Sensor (+/-0.5C): MCP9803
 I/O Expander 8/16-bit: MCP23016/MCP2308
Consider additionally:
 Battery gauges
 Audio codecs
 GPS receivers
 LCD displays controllers
 Card readers
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Application Specific Protocols
Examples of application specific protocols based
on the I2C interface are:
SMBus™, the System Management Bus, used in
personal computers (and servers) for low speed
system management applications
IPMI™, the Intelligent Platform Management
Interface, used to monitor the system health and
manage systems (mostly servers) remotely.
PMBus™, the Power Management Bus, used by
advanced digitally controlled power supply units to
monitor, configure and control power.
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
I2C Data Transfer Rules
Two simple rules dictate how to operate from here:
 When the SCL line is low, and only at this time, the SDA line can change.
 When the SCL line is high, the SDA line status indicates the value of a bit.
Two exceptions to rule 1 create special conditions that are used to delimit the
beginning and end of each transaction between two devices on the bus. When
SCL is high:
 A START condition is indicated by the SDA line changing from high to low
 A STOP condition is indicated by the SDA line changing from low to high
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
The 24LC00 Serial EEPROM
I2C has been for years the favorite choice for serial EEPROM users and
manufacturers for two reasons:
 Only two pins (I/Os) are required to communicate with the device,
enabling the embedded control designer to use very low pin count
(inexpensive) microcontrollers.
 Just four pads (areas of contacts on the chip) are required for a Serial
EEPROM device. Two provide the power supply and the remaining two
are the I2C lines. This reduces the silicon area overhead due to the
contacts on a typically very small device -- hence the high impact/cost
of the pads.
In practice most I2C serial EEPROMs have at least a fifth contact/pin (WP)
to protect the device contents by disabling writing.
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Talking to the Serial EEPROM
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
The SEE Grammar
Using the following simple notation:

S = START sequence
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P = STOP sequence
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A/N = Acknowledge bit
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0xXX= data byte (hex notation)
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0bXXXXXXXX =data byte (in binary notation)

and using brackets () to indicate parts of the conversation produced by a slave we can
represent a typical I2C protocol message in a compact notation
Here is an example of a read command sequence for a 24LC00 (128 bit) SEE:

S 0b01010001 (A) (Data) NP
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
A Write Example

S 0b01010000 (A) ByteAddress (A) Data (A) P
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
I2CxCON register
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Initializing the I2C module
void InitSEE( long fcy)
// fcy = processor operating frequency in Hz (system clock)
{ // Configure I2C for 7 bit address mode 100kHz
OpenI2C1( I2C_ON | I2C_IDLE_CON | I2C_7BIT_ADD | I2C_STR_EN
| I2C_GCALL_DIS | I2C_SM_DIS | I2C_IPMI_DIS,
(fcy /(2*BUS_FRQ))-1);
IdleI2C1();
T1CON=0x8030;
TMR1=0;
while( TMR1< 100);
} //InitSEE
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Sending the Address
int addressSEE( long add)
// send the address selection command
// repeat if SEE busy
{
int cmd;
// 1. Form SEE command + address msb (3)
cmd= 0xA0|((add>>7)&0xE);
// 2. WRITE(!) the Address msb
// try send command and repeat until ACK is received
while( 1)
{
StartI2C1();
IdleI2C1();
// send command and address msb(3)
MasterWriteI2C1( cmd+WRITE_CMD);
IdleI2C1();
if (I2C1STATbits.ACKSTAT==0)
break;
StopI2C1();
IdleI2C1();
} // while waiting for ACK
// 3. send byte address
MasterWriteI2C1( add);
IdleI2C1();
// 4. exit returning the cmd byte
return cmd;
} // addressSEE
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Logic Analyzer Capture
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Writing a 16-bit value
S 0b01010000 (A) ByteAddress (A) DataLSB (A) DataMSB (A) P
void iWriteSEE( long add, int v)
// SEE write command sequence
{
int cmd;
// 1. select address
cmd = AddressSEE( add);
// 2. stream data out
MasterWriteI2C1( v&0xFF);
IdleI2C1();
MasterWriteI2C1( v>>8);
IdleI2C1();
// 3. terminate the command sequence
StopI2C1();
IdleI2C1();
} // iWriteSEE
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Reading a 16-bit value
S 0b01010000 (A) ByteAddress(A) P
S 0b01010001 (A) (DataLSB) [A] (Data MSB) NP
int iReadSEE( long add)
// random access read command sequence
{
int cmd, r;
// 1. select address
cmd = AddressSEE( add);
StopI2C1();
IdleI2C1();
// 2. read command
StartI2C1(); IdleI2C1();
MasterWriteI2C1( cmd+READ_CMD);
IdleI2C1();
// 3. stream data in (will continue until NACK is sent)
r= MasterReadI2C1( );
AckI2C1(); IdleI2C1();
r|= (MasterReadI2C1()<<8);
// 4. terminate read sequence (send NACK then STOP)
NotAckI2C1(); IdleI2C1();
StopI2C1(); IdleI2C1();
return r;
} // iReadSEE
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Address Selection
Data Read
I2C Serial EEPROM Summary
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Notes for the PIC MCU Experts
In addition to the SPI options available on most PIC
microcontroller (offered by the SSP and MSSP modules), such as:
 Selectable clock polarity
 Selectable clock edge
 Master or Slave mode operation
The PIC24 SPI interface module adds several new capabilities,
including:
 A 16-bit transfer mode
 Data input sampling phase selection
 Framed transmission mode
 Frame synch pulse control (polarity and edge selectable)
 Enhanced Mode (8 deep transmit and receive FIFOs)
Using the new Enhanced Mode, with 8 levels deep FIFOs , up to 8
words of data (16 bytes) can be written or retrieved from the SPI
buffers in short bursts, leaving more time to the CPU to process
the data in between.
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Tips and Tricks
Safety measures suggested to reduce the risk of SEE data
corruption:
 Ensure adequate power supply decoupling (a capacitor)
is provided close to the memory device.
 A pull up resistor (10k Ohm) is provided on the Chip
Select line, to avoid floating during the microcontroller
power up and reset.
 An additional pull down resistor (10k Ohm) can be
provided on the SCK clock line to avoid clocking of the
peripheral during boundary scan and other board testing
procedures.
 Verify clean and fast power-up and down slope are
provided to the microcontroller to guarantee reliable
Power On Reset operation. If necessary add an external
voltage supervisor (see MCP809 devices for example).
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Tips and Tricks
A number of software methods can be employed to prevent a program
bug might trigger the write routine:
 Avoid reading and especially updating the NVM content right after
power up. Allow a few milliseconds for the power supply to stabilize
(application dependent).
 Add a software write-enable flag, and demand that the calling
application set the flag before calling the write routine, possibly after
verifying some essential application specific entry condition.
 Add a stack level counter; each function in the stack of calls
implemented by the library should increments the counter upon entry
and decrement it on exit. The write routine should refuse to perform if
the counter is not at the expected level.
 Some refuse to use the NVM memory locations corresponding to the
first address (0x0000) and/or the last address (0xffff) believing they
could be more likely to be the subject to corruption.
 Store two copies of each essential piece of data, performing two
separate calls to the write routine. If each copy contains even a simple
checksum, it will be easy, when reading it back, to discard the
corrupted one and recover.
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Suggested Excercises
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Several functions in the library are performing locking loops that could
reduce the overall application performance. By utilizing the SPI port
interrupts implement a non blocking version of the library.
Enable the new SPI 16-bit mode to accelerate basic read and write
operation.
Develop (circular) buffered versions of the read and write functions.
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Recommended Readings


Eady, F. (2004), Networking and
Internetworking with Microcontrollers,
Newnes, Burlinghton, MA
An entertaining introduction to serial
communication in embedded control.
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Online Resources

Total Endurance Software
http://www.microchip.com/totalendurance
 This tool will help you estimate the endurance to
expect from a given NVM device in your actual
application conditions. It will give you an indication of
the total number of e/w cycles or the number of
expected years of your application life before a certain
target failure-rate is reached.

Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)