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 2000 Microchip Technology Inc. Preliminary DS00735A-page 1

INTRODUCTION

This application note describes the implementation of

the PICmicro MSSP module for Master I2C communi-

cations. The Master Synchronous Serial Port (MSSP)

module is the enhanced Synchronous Serial Port

developed by Microchip Technology and is featured on

many of the PICmicro devices. This module provides

for both the 4-mode SPI communications, as well as

Master and Slave I2C communications, in hardware.

For information on the SPITM peripheral implementation

see the PICmicroTM Mid-Range MCU Family Reference

Manual, document DS33023. The MSSP module in I

mode fully implements all Master and Slave functions

(including general call support) and provides interrupts

on START and STOP bits in hardware to determine a

free I2C bus (multi-master function). The MSSP module

implements the standard mode specifications, as well

as 7-bit and 10-bit addressing. Figure 1 depicts a func-

tional block diagram of the I2C Master mode. The appli-

cation code for this I2C example is developed for and

tested on a PIC16F873, but can be ported over to a

PIC17CXXX and PIC18CXXX PICmicro MCU which

features a MSSP module.

FIGURE 1: I2C MASTER MODE BLOCK DIAGRAM

Author: Richard L. Fischer

Microchip Technology Inc.

Read Write

SSPSR

START bit, STOP bit,

SSPBUF

Internal

Data Bus

Set/Reset, S, P, WCOL (SSPSTAT)

Shift

Clock

MSb LSb

SDA

Acknowledge

Generate

SCL

SCL In

Bus Collision

SDA In

Receive Enable

Clock cntl

Clock Arbitrate/WCOL Detect

(hold off clock source)

SSPADD<6:0>

Baud

Set SSPIF, BCLIF

Reset ACKSTAT, PEN (SSPCON2)

Rate

Generator

SSPM3:SSPM0

START bit detect

STOP bit detect

Write collision detect

Clock Arbitration

State counter for

end of XMIT/RCV

AN735

Using the PICmicro® MSSP Module for Master

I2CTM Communications

AN735

DS00735A-page 2 Preliminary  2000 Microchip Technology Inc.

THE I2C BUS SPECIFICATION

Although a complete discussion of the I

2C bus specifi-

cation is outside the scope of this application note,

some of the basics will be covered here. For more infor-

mation on the I2C bus specification, you may refer to

sources indicated in the References section.

The Inter-Integrated-Circuit, or I2C bus specification

was originally developed by Philips Inc. for the transfer

of data between ICs at the PCB level. The physical

interface for the bus consists of two open-collector

lines; one for the clock (SCL) and one for data (SDA).

The SDA and SCL lines are pulled high by resistors

connected to the VDD rail. The bus may have a one

Master/many Slave configuration or may have multiple

master devices. The master device is responsible for

generating the clock source for the linked Slave

devices.

The I2C protocol supports either a 7-bit addressing

mode, or a 10-bit addressing mode, permitting 128 or

1024 physical devices to be on the bus, respectively. In

practice, the bus specification reserves certain

addresses so slightly fewer usable addresses are avail-

able. For example, the 7-bit addressing mode allows

112 usable addresses. The 7-bit address protocol is

used in this application note.

All data transfers on the bus are initiated by the master

device and are done eight bits at a time, MSb first.

There is no limit to the amount of data that can be sent

in one transfer. After each 8-bit transfer, a 9th clock

pulse is sent by the master. At this time, the transmit-

ting device on the bus releases the SDA line and the

receiving device on the bus acknowledges the data

sent by the transmitting device. An ACK (SDA held low)

is sent if the data was received successfully, or a NACK

(SDA left high) is sent if it was not received success-

fully. A NACK is also used to terminate a data transfer

after the last byte is received.

According to the I2C specification, all changes on the

SDA line must occur while the SCL line is low. This

restriction allows two unique conditions to be detected

on the bus; a START sequence (S) and a STOP

sequence (P). A START sequence occurs when the

master device pulls the SDA line low while the SCL line

is high. The START sequence tells all Slave devices on

the bus that address bytes are about to be sent. The

STOP sequence occurs when the SDA line goes high

while the SCL line is high, and it terminates the trans-

mission. Slave devices on the bus should reset their

receive logic after the STOP sequence has been

detected.

The I2C protocol also permits a Repeated Start condi-

tion (Rs), which allows the master device to execute a

START sequence without preceding it with a STOP

sequence. The Repeated Start is useful, for example,

when the Master device changes from a write operation

to a read operation and does not release control of the

bus.

MSSP MODULE SETUP,

IMPLEMENTATION AND CONTROL

The following sections describe the setup, implemen-

tation and control of the PICmicro MSSP module for

I2C Master mode. Some key Special Function Regis-

ters (SFRs) utilized by the MSSP module are:

1. SSP Control Register1 (SSPCON1)

2. SSP Control Register2 (SSPCON2)

3. SSP Status Register (SSPSTAT)

4. Pin Direction Control Register (TRISC)

5. Serial Receive/Transmit Buffer (SSPBUF)

6. SSP Shift Register (SSPSR) - Not directly

accessible

7. SSP Address Register (SSPADD)

8. SSP Hardware Event Status (PIR1)

9. SSP Interrupt Enable (PIE1)

10. SSP Bus Collision Status (PIR2)

11. SSP Bus Collision Interrupt Enable (PIE2)

Module Setup

To configure the MSSP module for Master I2C mode,

there are key SFR registers which must be initialized.

Respective code examples are shown for each.

1. SSP Control Register1 (SSPCON1)

• I2C Mode Configuration

2. SSP Address Register (SSPADD<6:0>)

• I2C Bit Rate

3. SSP Status Register (SSPSTAT)

• Slew Rate Control

• Input Pin Threshold Levels (SMbus or I2C)

4. Pin Direction Control (TRISC)

• SCL/SDA Direction

To configure the MSSP module for Master I2C mode,

the SSPCON1 register is modified as shown in

Example 1.

EXAMPLE 1: I2C MODE CONFIGURATION

With the two-wire synchronous I2C bus, the Master

generates all clock signals at a desired bit rate. Using

the formula in Equation 1, the bit rate can be calculated

and written to the SSPADD register. For a 400kHz bit

rate @ Fosc = 16MHz, the SSPADD register is modi-

fied as shown in Example 2.

movlw b’00101000’ ; setup value

; into W register

banksel SSPCON1 ; select SFR

; bank

movwf SSPCON1 ; configure for

; Master I 2C

AN735

 2000 Microchip Technology Inc. Preliminary DS00735A-page 3

EQUATION 1: BIT RATE CALCULATION

EXAMPLE 2: I2C BIT RATE SETUP

To enable the slew rate control for a bit rate of 400kHz

and select I2C input thresholds, the SSPSTAT register

is modified as shown in Example 3.

EXAMPLE 3: SLEW RATE CONTROL

The SSPSTAT register also provides for read-only

status bits which can be utilized to determine the status

of a data transfer, typically for the Slave data transfer

mode. These status bits are:

•D/A - Data/Address

•P - STOP

•S - START

•R/W - Read/Write Information

•UA - Update Address (10-bit mode only)

•BF - Buffer Full

Finally, before selecting any I2C mode, the SCL and

SDA pins must be configured to inputs by setting the

appropriate TRIS bits. Selecting an I2C mode by set-

ting the SSPEN bit (SSPCON1<5>), enables the SCL

and SDA pins to be used as the clock and data lines in

I2C mode. A logic "1" written to the respective TRIS bits

configure these pins as inputs. An example setup for a

PIC16F873 is shown in Example 4. Always refer to the

respective data sheet for the correct SCL and SDA

TRIS bit locations.

EXAMPLE 4: SCL/SDA PIN DIRECTION

SETUP

The four remaining SFR’s can be used to provide for

I2C event completion and Bus Collision interrupt func-

tionality.

1. SSP Event Interrupt Enable bit (SSPIE)

2. SSP Event Status bit (SSPIF)

3. SSP Bus Collision Interrupt Enable bit (BCLIE)

4. SSP Bus Collision Event Status bit (BCLIF)

Implementation and Control

Once the basic functionality of the MSSP module is

configured for Master I2C mode, the remaining steps

relate to the implementation and control of I

2C events.

The Master can initiate any of the following I

2C bus

events:

1. START

2. Restart

3. STOP

4. Read (Receive)

5. Acknowledge (after a read)

•Acknowledge

•Not Acknowledge (NACK)

6. Write

The first four events are initiated by asserting high the

appropriate control bit in the SSPCON2<3:0> register.

The Acknowledge bit event consists of first setting the

Acknowledge state, ACKDT (SSPCON2<5>) and then

asserting high the event control bit ACKEN

(SSPCON2<4>).

Data transfer with acknowledge is obligatory. The

acknowledge related clock is generated by the Master.

The transmitter releases the SDA line (HIGH) during

the acknowledge clock pulse. The receiver must pull

down the SDA line during the acknowledge clock pulse

so that it remains stable LOW during the HIGH period

of this clock pulse. This sequence is termed "ACK" or

acknowledge.

When the Slave doesn’t acknowledge the Master dur-

ing this acknowledge clock pulse (for any reason), the

data line must be left HIGH by the Slave. This

sequence is termed "NACK" or not acknowledge.

Example 5 shows an instruction sequence which will

generate an acknowledge event by the Master.

EXAMPLE 5: ACKNOWLEDGE EVENT

movlw b’00001001’ ; setup value

; into W register

banksel SSPADD ; select SFR bank

movwf SSPADD ; baud rate =

; 400KHz @ 16MHz

movlw b’00000000’ ; setup value

; into W register

movwf SSPSTAT ; slew rate

; enabled

banksel SSPSTAT ; select SFR bank

movlw b’00011000’ ; setup value

; into W register

banksel TRISC ; select SFR bank

iorwf TRISC,f ; SCL and SDA

; are inputs

SSPADD =

FOSC

Bit Rate

- 1

(

)

banksel SSPCON2 ; select SFR

; bank

bcf SSPCON2, ACKDT ; set ack bit

; state to 0

bsf SSPCON2, ACKEN ; initiate ack

Produktspezifikationen

Marke:	Microchip
Kategorie:	Nicht kategorisiert
Modell:	TC1321

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