Freescale Semiconductor
Application Note
Document Number: AN4481
Rev 0.1, 07/2012
Sensor I2C Setup and FAQ
by: Miguel Salhuana
1
Introduction
This document outlines how an MCU interfaces with a
Freescale sensor using the I2C communication protocol.
It is important to clearly understand the correct method
for setting up the I2C for communication between a
Freescale sensor and the MCU. All programming of the
Freescale sensor registers is done through the I2C
protocol as defined by the Philips I2C-Bus Specification
version 2.1, slave mode. This document assumes that the
user is familiar with I2C protocol and discusses I2C setup
for communication with the Freescale sensor. “FAQ ” on
page 10 reviews the most frequently asked questions of
the I2C interface.
This document covers the following topics:
• Configuring the I2C Address
• Single-Byte Write
• Multiple-Byte Write
• Single-Byte Read
• Multiple-Byte Read
• Bus Reset
• FAQ
• Sample Driver Code for Freescale
Microprocessors
© 2012 Freescale Semiconductor, Inc. All rights reserved.
Contents
1
2
3
4
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Keywords. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
I2C Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1 Explanation of Dummy Read. . . . . . . . . . . . . . . . . . 2
2.2 I2C Device Address . . . . . . . . . . . . . . . . . . . . . . . . 2
2.3 Single-Byte Write. . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.4 Multiple-Byte Write . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.5 Single-Byte Read. . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.6 Multiple-Byte Read . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.7 Bus Reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Sample Driver Code for Freescale Microprocessors. . . 13
4.1 IIC_Start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.2 IIC_Stop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3 IIC_RepeatStart. . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.4 IIC_CycleWrite. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.5 IIC_CycleRead . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.6 IIC_StopRead . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.1
Features
The I2C features, for Freescale sensors, include:
• Compliance with the Philips I2C-Bus Specification version 2.1
• Slave-only operation
• Single- and Multiple-Byte Write/read with auto-increment addressing capability.
• 7-bit addressing mode with 8-bit data retrieval
1.2
Keywords
I2C, IIC, Interrupt, Sensor, Streaming Data, Polling, Slave, Protocol
2
I2C Configuration
Please note that this document makes reference to the I2C as well as IIC.
• IIC - refers to the host/MCU controller
• I2C - refers to the slave/sensor controller
An example is the IIC host controller data register which has the name IICD. Therefore, when referring to
the slave/sensor we will use I2C and when referring to the host/MCU controller we will use IIC.
2.1
Explanation of Dummy Read
Please note that during the explanation of the algorithm we will make reference to a “Dummy Read”. This
read is not a true read of the I2C bus, but is a read of the IICD register in the IIC block. It will not be seen
on the bus or in any I2C specification because it is internal to the design of the I2C hardware block.
The I2C hardware block includes a state machine that will perform an IIC byte transfer on the bus each
time the IICD register is written to or read. The first byte (I2C device address) is sent out when the I2C
address value is written to the IICD register. Once this has been written out then it needs the Dummy Read
to initiate the transfer of the next byte. Note that it is a Dummy Read because there is no real data in the
IICD register yet because the hardware has not actually done the transfer.
Throughout the text of this document you will see references to the “Dummy Read” this is an artifact of
the implementation of the I2C controller block in the host MCU.
2.2
I2C Device Address
Each slave is assigned an I2C device address. The address consists of 8 bits, the 7 most-significant bits
represent the actual device address and the least significant bit is the read/write toggle. Please refer to the
device data sheet for the specific I2C address.
For example the MPL3115A2 sensor uses the standard 7-bit I2C slave address of 0x60 or 1100000. The
8-bit I2C write address is 0xC0 and the 8-bit read address is 0xC1.
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2.3
Single-Byte Write
In order to perform a single-byte write, use the following steps:
1. Setup START condition (a HIGH to LOW transition of SDA while SCL is HIGH)
2. Send I2C device address (last bit is 0 to write)
3. Send I2C register address
4. Write data
5. Send STOP (a LOW to HIGH transition of SDA while SCL is HIGH)
Please note that the START and STOP conditions are always generated by the master.
1
2
3
5
4
Single Byte Write
MASTER
ST Device Address [6:0]
SLAVE
W
Register Address [7:0]
AK
Data [7:0]
AK
SP
AK
Figure 1. Single-Byte Write
Example 1. Single-Register Write code example
/*********************************************************\
* IIC Write Register
* address - I2C Address
* reg - register address
* val - value to be written
\*********************************************************/
void IIC_RegWrite(byte address, byte reg, byte val)
{
IICC_TX = 1;
// Transmit Mode
IIC_Start();
// Send Start
IIC_CycleWrite(address);
// Send IIC "Write" Address
IIC_CycleWrite(reg);
// Send Register
IIC_CycleWrite(val);
// Send Value
IIC_Stop();
// Send Stop
}
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2.4
Multiple-Byte Write
When doing a multiple-byte write, we use the auto-incrementing address feature of the ASIC embedded
with the sensor. The steps to doing a multiple-byte read are:
1. Setup START condition (a HIGH to LOW transition of SDA while SCL is HIGH)
2. Send I2C device address (last bit is 0 to write)
3. Send register address
4. Write data (using a loop)
5. Send STOP (a LOW to HIGH transition of SDA while SCL is HIGH)
1
2
3
5
4
Multiple Byte Write
MASTER
ST Device Address [6:0]
W
Register Address [7:0]
AK
Data [7:0]
AK
Data [7:0]
AK
SP
AK
SLAVE
Figure 2. Multiple-Byte Write
Example 2. Multiple-Write Register code example
/*********************************************************\
* IIC Write Multiple Registers
* address - I2C Address
* reg1 - register address
* N
- number of bytes to write
* array - values to be written
*
\*********************************************************/
void IIC_RegWriteN(byte address, byte reg1,byte N,byte *array)
{
IICC_TX = 1;
// Transmit Mode
IIC_Start();
// Send Start
IIC_CycleWrite(address);
// Send IIC "Write" Address
IIC_CycleWrite(reg1);
// Send Register
while (N>0)
// Send N Values
{
IIC_CycleWrite(*array);
array++;
N--;
}
IIC_Stop();
// Send Stop
}
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2.5
Single-Byte Read
To do a single-byte read we use the following steps:
1. Setup START condition (HIGH to LOW transition of SDA while SCL is HIGH)
2. Send I2C device address (last bit is 0 to write)
3. Send I2C register address
4. Repeated START (HIGH to LOW transition of SDA while SCL is HIGH)
5. Send I2C device address (last bit set to read)
6. Dummy Read (internal only)
7. Read data
8. Send STOP (a LOW to HIGH transition of SDA while SCL is HIGH)
Please note that the START and STOP conditions are always generated by the master.
1
2
3
4
5
8
Single Byte Read
ST Device Address [6:0]
W
Register Address [7:0]
AK
SR Device Address[6:0]
R
NAK SP
AK
AK
Data [7:0]
7
Figure 3. Single-Byte Read
Example 3. Single-Read Register code example
/*********************************************************\
* IIC Read Register
* address - I2C Address
* reg - register address
\*********************************************************/
byte IIC_RegRead(byte address, byte reg)
{
byte b;
IICC_TX = 1;
// Transmit Mode
IIC_Start();
// Send Start
IIC_CycleWrite(address);
// Send IIC "Write" Address
IIC_CycleWrite(reg);
// Send Register
IIC_RepeatStart();
// Send Repeat Start
IIC_CycleWrite(address+1);
// Send IIC "Read" Address
b = IIC_CycleRead(1);
// Dummy read starts read transaction
b = IIC_CycleRead(1);
// Read Register Value
IIC_Stop();
// Send Stop
return b;
}
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2.6
Multiple-Byte Read
The multiple-byte read uses the sensor’s built in auto-address increment feature. This allows the host
controller to cycle through a specific range of registers by only sending the first register's address and then
reading the data for the number of registers specified in the function call. This address table is specific for
each sensor so please check the data sheet for the specified ranges.
To illustrate the use of the auto-address increment feature, we can take a look at the MPL3115A2 data sheet
register address map:
Table 1. Register Address Map
Reset
when
Reset
STBY
to Active
Register
Address
Name
0x00
Sensor Status Register
(STATUS)(1)(2)
0x01
Pressure Data Out MSB
(OUT P_MSB)(1)(2)
0x00
Yes
R
0x02
Pressure Data Out CSB
(OUT_P_CSB)(1)(2)
0x00
Yes
R
0x03
Bits 4-11 of 20-bit real-time Pressure sample
0x03
Pressure Data Out LSB
(OUT_P _LSB)(1)(2)
0x00
Yes
R
0x04
Bits 0-3 of 20-bit real-time Pressure sample
0x04
Temperature Data Out MSB
(OUT_T_MSB)(1)(2)
0x00
Yes
R
0x05
Bits 4-11 of 12-bit real-time Temperature sample
0x05
Temperature Data Out LSB
(OUT_T _LSB)(1)(2)
0x00
Yes
R
0x00
Bits 1-3 of 12-bit real-time Temperature sample
0x06/0x00
Sensor Status Register
(DR_STATUS)(1)(2)
0x00
Yes
R
0x07
Data Ready status information
0x00
Yes
Type
Auto-Increment
Address
Comment
R
0x01
Alias for DR_STATUS or F_STATUS
0x02
0x01
Bits 12-19 of 20-bit
real-time Pressure
sample.
Root pointer to
Pressure and
Temperature FIFO
data.
1. Register contents are preserved when transitioning from “ACTIVE” to “STANDBY” mode.
2. Register contents are reset when transitioning from “STANDBY” to “ACTIVE” mode.
As you can see in this table you can continually read the status and output register of the MPL3115A2 by
initiating a read at register 0x00 and it will auto-increment through the output registers and loop back to
the status register to begin the loop again.
The steps to doing a multiple-byte read are:
1. Setup START condition (HIGH to LOW transition of SDA while SCL is HIGH)
2. Send I2C device address (last bit is 0 to write)
3. Send register address
4. Repeated START (HIGH to LOW transition of SDA while SCL is HIGH)
5. Send I2C device address (last bit set to read)
6. Dummy Read (internal only)
7. Read data (using a loop)
8. Send STOP (a LOW to HIGH transition of SDA while SCL is HIGH)
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1
2
3
5
4
Multiple Byte Read
MASTER
ST Device Address [6:0]
W
...
Register Address [7:0]
SR Device Address[6:0]
R
AK
....
AK
AK
SLAVE
AK
8
Data [7:0]
7
...
AK
MASTER
...
SLAVE
Data [7:0]
AK
Data [7:0]
NAK SP
Data [7:0]
Figure 4. Multiple-Byte Read
Example 4. Multiple-Read Register code example
/*********************************************************\
* IIC Read Multiple Registers
* address - I2C Address
* reg1 - register address
* N
- number of bytes to read
* array - values to be read
\*********************************************************/
void IIC_RegReadN(byte address, byte reg1, byte N, byte *array)
{
byte b;
IICC_TX = 1;
// Transmit Mode
IIC_Start();
// Send Start
IIC_CycleWrite(address);
// Send IIC "Write" Address
IIC_CycleWrite(reg1);
// Send Register
IIC_RepeatStart();
// Send Repeat Start
IIC_CycleWrite(address+1);
// Send IIC "Read" Address
b = IIC_CycleRead(0);
// Dummy read
while (N>1)
// Read N-1 Register Values
{
b = IIC_CycleRead(0);
*array = b;
array++;
N--;
}
b = IIC_CycleRead(1);
*array = b;
// Read Last value
IIC_Stop();
// Send Stop
}
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2.7
Bus Reset
If the I2C master stops in the middle of an I2C transaction, to a slave, then it may leave the slave device in
an incomplete state which can prevent future transactions from succeeding. Many slave devices may not
have a reset pin due to pin limitation therefore, in order to recover from this condition the I2C bus on the
slave can be reset by bit banging the bus or by power cycling the system and/or the slave devices.
The sequence for a Bus Reset is as follows:
• Disable the host MCU IIC controller
• Create a START condition
• Clock SCL for at least nine clocks
• Check for SDA high
• Create a STOP condition
• Enable the host MCU IIC controller
Example 5. Bus Reset code example
/*********************************************************\
* Initiate IIC Bus Reset
*
*
The transitions here are controlled through the data direction pin
*
of the MCU. When writing a 1 to the data direction (DD) pin of the
*
MCU control register this makes the pin into an output and is driven
*
low since the data register is set to 0.
*
When a 0 is written to the DD bit, the pin is set as an input (floating)
*
which is pulled high by the external pullup resistors on the I2C lines.
*
Please see Application Note AN4481 FAQ for more information.
\*********************************************************/
void IIC_Bus_Reset(void)
{
int loop;
// Disable the I2C block on the Host Controller
IICC1 &= ~(init_IICC1);
PAUSE;
/* Create START condition (SDA goes low while SCL is high) */
I2C_SDA_DD = 1; // SDA = 0
PAUSE;
I2C_SCL_DD = 0; // SCL = 1
PAUSE;
/* Release SDA back high */
I2C_SDA_DD = 0; // SDA = 1
PAUSE;
/* Clock SCL for at least 9 clocks until SDA goes high */
/* This loop is significantly greater than 9 clocks to */
/* make sure that this condition is met.
*/
loop = 0;
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while (loop < 100)
{
loop++;
/* Apply one SCL clock pulse */
I2C_SCL_DD = 1; // SCL = 1
PAUSE;
I2C_SCL_DD = 0; // SCL = 0
PAUSE;
/* If SDA is high and a complete byte was sent then exit the loop */
if ((I2C_SDA_PIN) && ((loop % 9) == 0))
break;
}
/* Create STOP condition (SDA goes high while SCL is high) */
I2C_SDA_DD = 1; // SDA = 0
PAUSE;
I2C_SCL_DD = 0; // SCL = 1
PAUSE;
I2C_SDA_DD = 0; // SDA = 1
PAUSE;
//Set operation back to default for all pins on PTBDD and Enable I2C
PTBDD = init_PTBDD;
IICC1 = init_IICC1;
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3
FAQ
What are the pullup resistor values?
The normal recommended resistor value is 4.7 k however, for high-capacitive loads and fast busses, this
can be reduced to as low as 1 k.
VDD
I2C Device
I2C Device
Rp
Rp
SDA
SCL
Figure 5. I2C Resistors
For more information on how to calculate these values please refer to the Philips I2C-Bus Specification
version 2.1, section 16.1.
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What are the RC values that affect the value of the pullup resistors?
Many factors can be considered in choosing the correct value for a pullup resistor for the I2C clock and
data signals. There is an RC time constant on the bus, where the pullup resistor and bus capacitance are
the R and C values. Also entering into the equation are the bus clock rate, and the high-logic voltage level.
Lower voltage interfaces, 3.3V versus 5.0V, can run faster with the same value pullup resistor. Bus
capacitance is influenced by the type of devices on the bus as well as the physical length of the bus and the
type of PC board, ground planes etc.
The calculations can quickly become very complicated, but there are a few basic ideas that will help
determine if the resistor is suitable. In order create robust signaling on the I2C bus, the rise time of the clock
and data signals should be a small portion (say 10-20%) of the total clock / data period. I2C is a fully
asynchronous logic interface, where one of the signals happens to be named CLK, but the behavior is more
like a data valid, enable, or strobe. It is important in all asynchronous designs to make sure that the data
path and the enable path do not line up. The data signal should be stable (either high or low), then the clock
signal floats high then is driven low. The data line normally only changes while the clock is low. If the data
changes when clock is high, then that indicates a start (high to low transition) or stop (low to high
transition) condition. Here is an example waveform.
Figure 6. An example of an I2C Timing Waveform
Some useful rules of thumb:
– 4.7 k resistor is a good starting point, and most used for DC-100 kHz-400 kHz operation.
– For slave devices capable of running in the 1-5 MHz range, a 1 k resistor may be required.
– For extremely long I2C lines, for many slave devices, where the bus capacitance is in the
200 pF or more range, a 1 k resistor may be required.
– Resistors lower than 1 k are not recommended due to the current required to work against
a low-resistance load.
What is the maximum speed of the I2C?
All Freescale sensors support an I2C bus speed of 400 kHz. Faster speeds are possible for some sensors;
please refer to the sensor's data sheet for further information. The I2C bus should only run as fast as the
slowest device on the bus.
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What is Clock Stretching?
When the master is reading from the slave, it's the slave that places the data on the SDA line, but it's the
master that controls the clock. What if the slave is not ready to send the data? The sensor on the slave
device will need to go to an interrupt routine, save its working registers, find out what address the master
wants to read from, get the data and place it in its transmission register. This can take many µs to happen,
meanwhile, the master is blissfully sending out clock pulses on the SCL line that the slave cannot respond
to.
The I2C protocol provides a solution to this: the slave is allowed to hold the SCL line low. This is called
clock stretching. When the slave gets the read command from the master, it holds the clock line low. The
sensor then gets the requested data, places it in the transmission register and releases the clock line,
allowing the pullup resistor to finally pull it high. Well behaved master's will issue the first clock pulse of
the read by making SCL high and then check to see if it really has gone high. If it's still low then it's the
slave that holding it low and the master should wait until it goes high before continuing. Luckily, the
hardware I2C ports on most MCUs will handle this automatically.
I2C is not reading the correct data from the expected address
Please see “Explanation of Dummy Read” on page 2.
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4
Sample Driver Code for Freescale Microprocessors
The sample code was written for the demo DEMOSTB board which utilizes the Freescale MC9S08QE8
processor.
Figure 7. MPL3115A2 daughter board, LFSTBEBMPL3115A2 evaluation board and
the LSTBUS interface board
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The following acronyms, defined in the MC9S08QE8 Reference Manual, are used in the sample code.
These correspond to bits within the IIC control registers within the MCU:
Table 2. Acronym Definitions
Acronym
IICC_MST
Master Mode Select
IICS_BUSY
Bus Busy
IICC_RSTA
Repeat Start
IICS_TCF
Transfer Complete Flag
IICS_IICIF
IIC Interrupt Flag
IICS_RXAK
Receive Acknowledge
IICC_TX
Transmit Mode Select
IICC_TXAK
Transmit Acknowledge
IICD
4.1
Definition
Data Register
IIC_Start
/*********************************************************\
* Initiate IIC Start Condition
\*********************************************************/
void IIC_Start(void)
{
IICC_MST = 1;
timeout = 0;
while ((!IICS_BUSY) && (timeout<1000))
timeout++;
if (timeout >= 1000)
error |= 0x01;
} //*** Wait until BUSY=1
4.2
IIC_Stop
/*********************************************************\
* Initiate IIC Stop Condition
\*********************************************************/
void IIC_Stop(void)
{
IICC_MST = 0;
timeout = 0;
while ( (IICS_BUSY) && (timeout<1000))
timeout++;
if (timeout >= 1000)
error |= 0x02;
} //*** Wait until BUSY=0
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4.3
IIC_RepeatStart
/*********************************************************\
* Initiate IIC Repeat Start Condition
\*********************************************************/
void IIC_RepeatStart(void)
{
IICC_RSTA = 1;
timeout = 0;
while ((!IICS_BUSY) && (timeout<1000))
timeout++;
if (timeout >= 1000)
error |= 0x04;
} //*** Wait until BUSY=1
4.4
IIC_CycleWrite
/*********************************************************\
* IIC Cycle Write
\*********************************************************/
void IIC_CycleWrite(byte bout)
{
timeout = 0;
while ((!IICS_TCF) && (timeout<1000))
timeout++;
if (timeout >= 1000)
error |= 0x08;
IICD = bout;
timeout = 0;
while ((!IICS_IICIF) && (timeout<1000))
timeout++;
if (timeout >= 1000)
error |= 0x10;
IICS_IICIF = 1;
if (IICS_RXAK)
error |= 0x20;
}
4.5
IIC_CycleRead
/*********************************************************\
* IIC Cycle Read
\*********************************************************/
byte IIC_CycleRead(byte byteLeft)
{
byte bread;
timeout = 0;
while ((!IICS_TCF) && (timeout<1000))
timeout++;
if (timeout >= 1000)
error|=0x08;
IICC_TX = 0;
IICC_TXAK = byteLeft <= 1 ? 1 : 0; //Set NACK when reading the last byte
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bread = IICD;
timeout = 0;
while ((!IICS_IICIF) && (timeout<1000))
while ((!IICS_TCF) && (timeout<1000))
timeout++;
if (timeout >= 1000)
error|=0x08;
IICC_TX = 0;
IICC_TXAK = byteLeft <= 1 ? 1 : 0; //Set NACK when reading the last byte
bread = IICD;
timeout = 0;
while ((!IICS_IICIF) && (timeout<1000))
timeout++;
if (timeout) >= 1000)
error |= 0x10;
IICS_IICIF=1;
return bread;
4.6
IIC_StopRead
/*********************************************************\
* Initiate IIC Stop Condition on Read
\*********************************************************/
byte IIC_StopRead(void)
{
IICC_MST = 0;
timeout = 0;
while ( (IICS_BUSY) && (timeout<1000))
timeout++;
if (timeout >= 1000)
error |= 0x02;
} //*** Wait until BUSY=0
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All other product or service names are the property of their respective owners.
© 2012 Freescale Semiconductor, Inc. All rights reserved.
AN4481
Rev 0.1
07/2012