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Page 1
Application Report
SLAA115 ­ December 2000
1
I²C Interfacing of the MSP430 to a 24xx Series EEPROM
Brian Merritt
Mixed Signal Products
ABSTRACT
This application report and the associated software demonstrate how to interface an
MSP430 microcontroller to an external EEPROM memory using the I
2
C bus standard. The
circuit and software presented are adaptable to any MSP430 device, EEPROMs with
other memory sizes, or other external devices that also utilize an I
2
C interface.
Introduction
Many of the MSP430 microcontrollers either have Flash memory, which the application software
can erase and program in-circuit, or they have one time programmable (OTP) memory that
external sources can program in-circuit. Even so, there are applications where additional
external EEPROM memory is needed. In the case of Flash parts an EEPROM may be desired if
most of the MSP430's memory is required to store the program, or if a large amount of memory
is needed for data logging applications. When using OTP or ROM parts, EEPROM memory may
be needed to enable the application software to update calibration constants, change serial
numbers, or datalog sensor readings. The use of the I
2
C bus is a common method of interfacing
external EEPROM memories, and other devices, to microcontrollers.
The MSP430 microcontrollers are able to emulate the open collector outputs often used for I
2
C
interfacing. This application report demonstrates how to accomplish this by using an
MSP430x11x1 device to interface to, and communicate with, a 512 byte, 24LC04 EEPROM
memory device via I
2
C. No specific hardware modules of the MSP430 are required to implement
I
2
C, so this software can be easily adapted to any member of the product line. The example
circuit and associated software read and write to random memory locations of the external
EEPROM memory.
The software associated with this application report is designed to communicate to an EEPROM
memory. Since all I
2
C communications work in the same general way, the program could be
readily modified to communicate with other devices that also utilize the I
2
C bus interface. The I
2
C
portion of the code only requires about 420 bytes of program memory. The remainder of the
program supports a user interface via a PC. That portion of the code requires about 520 bytes of
program memory. The user interface is not required for I
2
C communications. It is included in the
program to facilitate the demonstration of the I
2
C code and hardware.
Page 2
SLAA115
2
I²C Interfacing of the MSP430 to a 24xx Series EEPROM
Theory of Operation
Using any two available I/O pins, the MSP430 is capable of emulating the open collector
operation of the SDA and SCL signals required for I
2
C communications. This is accomplished by
setting the output data for these two I/O pins low and using the directions setting of the pins to
determine the logic level of the signal lines. Whenever a logic high is required, the direction bit
for the corresponding pin is set to configure that pin as an input. The pullup resistor on the signal
line then pulls it to a logic high state. If a logic low is required, the direction bit is set to switch the
pin to an output. Since the data for the pins was previously set low, the signal line will be pulled
low by the pin being switched to an output.
The MSP430 initiates a write to a random byte of the EEPROM memory, by first sending a start
condition. Then the control byte is sent. The control byte consists of the device code for the
specific device that is being addressed, the address of the memory block being accessed, and a
bit that indicates whether a read or write operation is being performed (see Figure 1). The
EEPROM then acknowledges receipt of the byte. The MSP430 then sends the specific memory
address of the byte being written to. The EEPROM acknowledges again. The MSP430 then
sends the data to be written into the memory byte. After the EEPROM acknowledges the data,
the MSP430 sends a stop condition.
The reading of random bytes of the EEPROM memory is somewhat different than writing to
them. The main difference is that the control byte is sent twice, once before the MSP430 sends
the address byte and a second time before the EEPROM returns the requested data byte. The
read/write bit of the control word is also set to a logic high to indicate a read operation.
The associated software was written specifically to address the 512 bytes of memory in a
24LC04 EEPROM, but smaller memories would also work since the address bits in the control
byte would be ignored. Larger memories that utilize more of the address bits in the control byte
could also be supported if the software were modified to set the address bits as required.
The user interface is accomplished by communicating with a terminal program running on a PC.
Memory and data bytes are exchanged between the MSP430 and PC in the form of ASCII
characters. The terminal program runs at 9600 baud using an 8N1 protocol.
Figure 1. Control Byte for the 24LC04 EEPROM
1
0
1
0
X
X
A0
R/W
(A1)
(A2)
4-bit code for 24LC04
address of memory
block
read /
write
Note: X = don't care for 24LC04
Page 3
SLAA115
I²C Interfacing of the MSP430 to a 24xx Series EEPROM
3
Example Circuit
The example circuit in figure 2 is powered directly from the RS232 serial port of a PC. The interface
to the serial port is accomplished using two TI SN74AH1G04 inverters. If a fully compliant RS232
solution is required, the TI MAX3221 (3V) could be substituted. Two pullup resistors are needed on
the I
2
C signal lines to pull the lines high when the MSP430 I/Os are switched to inputs. A TI
TPS77033 low drop out regulator is also added to regulate the voltage level from the PC serial port.
Figure 2. Schematic of the I
2
C Bus and PC Serial Interfaces to the MSP430
Example Code
The example code for this application is available from the MSP430 web site,
http://www.ti.com/sc/msp430
, as a .zip file under this application note listing. The title of the program
is
I2C_EEmem.S43. The code is written in assembly language using the IAR Kickstart integrated
development environment.
References
1. MSP430x11x1 Data sheet (SLAS241C)
2. MSP430x1xx User's Guide (SLAU049)
3. 24xx Data sheets (various companies)
IN
OUT
EN GND
TPS77033
MSP430x11x(1)
XIN
XOUT
6
5
3
2
5
1
Vcc
RST/NMI
2
7
3
4
5
RX
TX
2
7
8
100K
10K
10K
24LC04
SDA
SCL
5
6
8
9
P2.0
P2.1
4
2
3
5
5
3
4
2
10
14
P1.1
P2.2
4
1
µ
f
4.7
µ
f
DB9
32,768 Hz
Vss
4
1K
Page 4
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