アプリケーションノート 826

Interfacing the MAX7651/MAX7652 12-Bit Data Acquisition System to the 24C02 2-Wire Serial EEPROM


要約: This article covers the specific hardware description and software routines required to interface the MAX7651 and MAX7652 12-bit data acquisition system to the 24C02 2-wire serial EEPROM. Detailed software code is provided. Since the MAX7651/52 is based on a standard 8051 processor core the information presented here is useful to any standard 8051-based design.

The 24Cxx series of 2-wire serial EEPROMs are widely used in 8051 microprocessor systems. Although the MAX7651/MAX7652 flash-programmable 12-bit data acquisition systems have 16K of internal flash memory, there are many "legacy" products that use small and inexpensive external memories.

This application note provides basic 2-wire WRITE and READ software subroutines. They can be easily modified to address additional features of EEPROMs, such as memory protection and bank addressing.

There are many derivatives of the 24C02 serial EEPROM, which include additional memory and page addressing. The 24C02 is widely used and is the part used in this example. Other derivative parts can use this code with minor modifications.

EEPROM Signals and Timing

The 24Cxx family uses two I/O lines for interfacing: SCL (Serial Clock) and SDA (Serial Data). SCL edges have different functions, depending on whether a device is being read from or written to. When clocking data into the device, the positive edges of the clock latch the data. The negative clock edges clock data out of the device.

The SDA signal is bi-directional, and is physically an open-drain so that multiple EEPROMs or other devices can share the pin. Both SCL and SDA must be pulled high externally.

The protocol used by the EEPROM is based in part on an ACK (acknowledge) bit sent by the EEPROM, if the data sent to it has been received. All addresses and data are sent in 8-bit words. The EEPROM sends the ACK as a low bit period during the ninth clock cycle. The EEPROM looks for specific transitions on the SCL and SDA pins to qualify READ and WRITE.

Data on the SDA pin may change only during the time SCL is low. Data changes during SCL high periods indicate a START or STOP condition. A START condition is a high-to-low transition of SDA with SCL high. All data transfers must begin with a START condition.

A STOP condition is a low-to-high transition of SDA with SCL high. All data transfers must end with a STOP condition. After a READ, the STOP places the EEPROM in a standby power mode. Refer to Figure 1 for START and STOP conditions.

Figure 1. START and STOP conditions.
Figure 1. START and STOP conditions.

Device Addressing

The 24C02 has 3 physical pins, designated A2, A1, and A0, which are tied to logic 1 or 0 levels. This allows eight unique hardware addresses, so that up to eight 24C02s can share the SCL and SDA lines without conflict. There is an internal address comparator that looks for a match between the address sent by the master controller and the 24C02's unique 7-bit address, determined in part by A2, A1, and A0. Refer to Table 1below.

Table 1. 24C02 Device Address
MSB
           
LSB
1
0
1
0
A2
A1
A0
R/~W

The device address is sent immediately after a START condition. The first four bits are the sequence "1010", which is a simple "noise filter" which prevents a random noise burst on the lines from accessing the device. The last bit sent is a 1 for READ and a 0 for WRITE. The code example below is for random READ/WRITE operations. The part can also perform Page Write/Sequential Read with slight code modifications. See the 24C02 data sheet for more information.

Byte Write to Memory

The Byte Write sequence is shown in Figure 2. After receiving a START condition and a device address, the EEPROM sends an ACK if the device address matches its own unique address. The MAX7651 waits for the ACK and aborts communication if it is not present. Next, an 8-bit byte address is sent, followed by another ACK. The MAX7651 then sends the 8-bit data byte, waits for the third ACK, and sends a STOP condition.

Figure 2. WRITE operation.
Figure 2. WRITE operation.

It is important to note that after the STOP condition is received, the EEPROM internally waits for the data to be stored into its internal memory array. This can take as long as 10ms. The 24C02 will ignore attempted accesses while the internal EEPROM is being programmed. The part can be polled for completion of the internal write cycle. This involves sending another START condition (also called a REPEATED START), followed by the device address byte. Note, in this case, there is no STOP condition sent. The EEPROM will send an ACK if the internal programming cycle is completed. The MAX7651 can also be programmed to wait 10ms before proceeding.

Byte Read from Memory

Reading a byte from the 24C02 EEPROM at a random address requires that a dummy WRITE operation be performed before the READ. See Figure 3.

The sequence is:
  • START condition
  • Send device address with R/~W = 0 'dummy WRITE' command
  • Wait for ACK
  • Send byte memory address
  • Wait for ACK
  • Send REPEATED START condition
  • Send device address with R/~W = 1 (READ command)
  • Wait for ACK
  • Read the 8 data bits into the MAX7651, MSB first
  • No ACK
  • STOP condition
This sequence is quite involved! The total number of SCL transitions required for a READ is 38.

Figure 3. READ operation.
Figure 3. READ operation.

Code Example

The following assembly language code example assumes a 24C02 EEPROM addressed at device 0H (i.e., A2 = A1 = A0 = ground). The MAX7651 uses two unused general-purpose I/O port pins to bit-bang the serial clock (SCL) and the bi-direction data line (SDA). Two internal RAM locations are needed: EE_ADDR stores the byte address and EE_DATA stores the data.

; EEPROM ROUTINES FOR THE 24C02, with A2 = A1 = A0 = 0

EE_WRITE: CALL EE_START ; SEND A START FLAG TO THE EEPROM
MOV A,#0A0H ; SPECIFY A WRITE EEPROM @ ADDRESS 0H
CALL SHOUT ; SHIFT OUT THE DEVICE ADDRESS
JC WR_ABORT ; ABORT IF NO ACK FROM EEPROM
MOV A,EE_ADDR ; GET EEPROM MEMORY ADDRESS
CALL SHOUT ; SHIFT OUT THE MEMORY ADDRESS
JC WR_ABORT ; ABORT IF NO ACK FROM EEPROM
MOV A, EE_DATA ; GET THE DATA TO BE WRITTEN
CALL SHOUT ; SHIFT OUT THE DATA
JC WR_ABORT ;
CLR C ;
WR_ABORT: CALL EE_STOP ; SEND STOP CONDITION TO EEPROM

; WAIT FOR WRITE TIME OF THE 24C02 {10ms}
; THE EEPROM TAKES 10ms TO INTERNALLY STORE THE DATA. YOU CAN EITHER
; PUT THE MICROCONTROLLER IN A WAIT STATE, OR CONTINUE WITH EXECUTION,
; KEEPING IN MIND THAT THE EEPROM DATA IS NOT STORED FOR 10ms!

RET ; GO BACK TO MAIN PROGRAM

; READ THE EEPROM DATA - FIRST PERFORM 'DUMMY WRITE'

EE_READ: MOV EE_DATA,#00H ; CLEAR OLD DATA
CALL EE_START ; SEND A START FLAG TO EEPROM
MOV A,#0A0H ; SPECIFY A WRITE TO EEPROM @ ADDRESS 0H
CALL SHOUT ; PERFORM 'DUMMY WRITE'
JC RD_ABORT ; ABORT IF NO ACK
MOV A,EE_ADDR ; LOAD EEPROM MEMORY LOCATION
  ; FROM WHICH TO READ
CALL SHOUT ; WRITE EEPROM MEMORY LOCATION
JC RD_ABORT ; ABORT IF NO ACK

; NOW READ THE DATA!

EE_WRITE: CALL EE_START ; SEND A START FLAG
MOV A,#0A1H ; SPECIFY A READ FROM EEPROM
CALL SHOUT ; SHIFT OUT EEPROM ADDRESS
JC RD_ABORT ; ABORT IF NO ACK
CALL SHIN ; SHIFT IN THE DATA FROM EEPROM
MOV EE_DATA,A ; STORE THE DATA
CALL NAK ; SEND A NAK (NO ACKNOWLEDGE) TO THE
; EEPROM
CLR C ; CLEAR ERROR FLAG
RD_ABORT: CALL EE_STOP ; ALL DONE
RET ;

; EE_START BIT-BANGS A START SEQUENCE TO EEPROM (HI-TO-LOW SDA TRANSITION
; WITH SCL HIGH).

EE_START: SETB SDA
SETB SCL ; SET BOTH BITS
NOP ; DELAY
CLR SDA ; START CONDITION; SDA HI TO LOW TRANSITION
NOP
NOP ; EEPROM ACCESS TIME DELAY
CLR SCL
CLR C ; CLEAR ERROR FLAG
RET ; ALL DONE

; EE_STOP SENDS A STOP SEQUENCE TO THE EEPROM (LOW-TO-HIGH SDA TRANSITION
; WITH SCL HIGH).

EE_STOP: CLR SDA  
NOP
NOP
SETB SCL
NOP
NOP ; SETUP TIME DELAY
SETB SDA ; SEND A STOP CONDITION
RET

; SHOUT SHIFTS DATA OUT TO THE EEPROM

SHOUT: PUSH B
  MOV B,#8 ; SAVE B AND LOAD BIT COUNT
EEOUT: RLC A ; SHIFT BIT LEFT (RLC=ROTATE LEFT THROUGH
; CARRY)
MOV SDA,C ; GET DATA BIT FROM CARRY
NOP
SETB SCL ; CLOCK IN 1-BIT
NOP ; CLOCK HIGH TIME
  CLR SCL ; CLOCK IS NOW LOW
DJNZ B,EEOUT ; DO IT 8 TIMES
SETB SDA ; RELEASE SDA FOR ACK
NOP
NOP
SETB SCL ; ACK CLOCK
NOP
MOV C,SDA ; GET THE ACK
CLR SCL ; CLEAR THE CLOCK BIT
POP B ; RESTORE WHATEVER B WAS
RET

; SHIN SHIFT DATA IN FROM THE EEPROM

SHIN: SETB SDA ; MAKE SDA AN INPUT
PUSH B
MOV B,#8 ; SAVE B AND SET BIT COUNTER
EEIN: NOP
SETB SCL ; SET CLOCK
NOP
NOP ; EEPROM ACCESS TIME
SETB SDA ; SET = 1 SO USED AS INPUT
MOV C,SDA ; READ 1-BIT
RLC A ; SHIFT BIT LEFT
CLR SCL ; CLEAR CLOCK BIT
DJNZ B,EEIN ; GET NEXT BIT IF LESS THAN 8 BITS READ
POP B
RET

; ACK SENDS AN EEPROM ACKNOWLDEGE

ACK: CLR SDA
NOP
NOP
SETB SCL ; CLOCK THE ACK
NOP
CLR SCL ; BRING CLOCK LOW
RET

; NAK SENDS A NO ACKNOWLEDGE

NAK: SETB SDA
NOP
NOP
SETB SCL ; CLOCK THE NAK
NOP
CLR SCL ; BRING CLOCK LOW
RET

$EJECT

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APP 826:
アプリケーションノート 826,AN826, AN 826, APP826, Appnote826, Appnote 826