reads, writes, and erases a 256-byte serial EEPROM.
One of the best features of the BASIC Stamp hardware is the 93LC56 serial EEPROM. The Stamp uses this device for program space, lookup tables, and nonvolatile data storage.
The program listing presents a series of subroutines that read, write, erase, write-protect, and write-enable the 93LC56. These routines are not port- and pin-independent. The program is based on the circuit shown in the illustration. If you need to change this circuit, make sure to change both the equates for D, CLK and CS, and the data-direction settings for the EEPROM port (mov !ra,x). These are at the start of the program, and in the subroutines EEread and Busy.
The program, which originally appeared in Parallax PIC Application Note #11, writes a rotating bit pattern to the EEPROM, then plays it back on a bank of eight LEDs connected to port RB. Not very exciting, but it verifies every storage cell of the EEPROM with a minimum of programming. Not all programs require the entire set of subroutines. Some are included just for the sake of demonstrating all the EEPROM's capabilities. For example, most programs can do without the routine that erases individual bytes, since a write accomplishes the same thing. And very few programs need to completely erase the EEPROM, or write a single value to all cells. So you should be able to remove EErase, EEwipe, and EEwrall to conserve program-memory space.
For more information on the EEPROM's characteristics and capabilities, please see the current edition of the Microchip Embedded Control Handbook.
To see EEPROM in operation, connect the circuit below to an erasable PIC or PIC emulator, such as the Parallax downloader. Assemble and run EEPROM.SRC. When you apply power to the PIC, it will write a scanning pattern to the bits of the EEPROM, then read this pattern back onto the LEDs.
;
; ***************************************************************************
; *** Bubble Software Parallax to PIC Source Converter. Copyright 1999. ***
; *** http://www.bubblesoftonline.com email: sales@picnpoke.com ***
; ***************************************************************************
;
; EEPROM (related subroutines)
; This is a collection of subroutines for reading, writing, and
; erasing the 93LC56 (or 'C56) serial EEPROM. As a demonstration, it
; writes a scanning pattern to the 256 bytes of the EEPROM, and then
; reads it back to eight LEDs connected to port rb.
P = pic16c54
#include <16c54.inc> ; processor assembler definitions
_CONFIG _xt_osc & _wdt_off & _protect_off
reset start
D equ ra.0 ; Pins DI and DO of the EEPROM
CLK equ ra.1 ; Clock pin--data valid on rising edge
CS equ ra.2 ; Chip select--high = active
ROP equ d'192' ; Opcode for read
WROP equ d'160' ; Opcode for write
EWEN equ d'152' ; Opcode to enable erase and write
EWDS equ d'128' ; Opcode to disable erase and write
ERAS equ d'224' ; Opcode to erase a byte
ERAL equ d'144' ; Opcode to erase all bytes
WRAL equ d'136' ; Opcode to write all bytes
org 8
temp Res d'1' ; Temporary variable for EE routines
EEdata Res d'1' ; Data to EEwrite/wrall, from EEread
EEaddr Res d'1' ; Address to EErase, EEwrite, EEread
clocks Res d'1' ; Number of clock cycles for SHiftOUT
tick1 Res d'1' ; Timer for demo.
tick2 Res d'1' ; Timer for demo.
org 0
start MOVLW d'0' ; Clear ports
MOVWF 5h
MOVLW d'0'
MOVWF 6h
MOVLW d'0' ; Make all ra pins outputs initially.
TRIS 5h
MOVLW d'0' ; Output for LEDs.
TRIS 6h
CALL EEnable ; Turn off write/erase protection.
MOVLW d'1' ; EEdata = 1.
MOVWF EEdata
MOVLW d'0' ; Start at address 0.
MOVWF EEaddr
start_loop CALL EEwrite ; Write pattern to EEPROM
CALL Busy ; Wait until done writing.
RRF EEdata ; Rotate the pattern right.
INCFSZ EEaddr ; FOR EEaddr = 0 to 255...
GOTO start_loop
CALL EEdisbl ; Turn write/erase protection back on.
start_loop2 CALL EEread ; Now read back contents of EEPROM.
MOVF EEdata,w ; Display the EEPROM data on LEDs.
MOVWF 6h
INCF EEaddr ; Set up for next address.
CALL delay ; Delay to slow down the scanning.
GOTO start_loop2 ; Do forever.
; Shout shifts out the bits of the variable temp to the EEPROM data line.
; All of the other EEPROM routines call Shout.
Shout RLF temp ; Rotate bit7 of temp into carry
BTFSS status,c ; Move carry bit to input of EEPROM
BCF 5h,d'0'
BTFSC status,c
BSF 5h,d'0'
BSF 5h,d'1' ; Clock the bit into EEPROM
NOP ; Clock must be high > 500 ns
BCF 5h,d'1'
DECFSZ clocks ; FOR I = 1 TO clocks...
GOTO Shout
RETLW 0h
; Read the byte in EEaddr into EEdata.
EEread MOVLW ROP ; Move the read opcode into temp
MOVWF temp
MOVLW d'4' ; Number of bits to shift out (op+1)
MOVWF clocks
BSF 5h,d'2' ; Select the EEPROM.
CALL Shout ; Shift the read opcode out.
MOVLW d'8' ; Number of bits to output (8 addr bits).
MOVWF clocks
MOVF EEaddr,w ; Set up to send the address.
MOVWF temp
CALL Shout ; Shift out the address.
MOVLW d'1' ; Now, change D to input.
TRIS 5h
MOVLW d'8' ; Number of bits to input (8 data bits).
MOVWF clocks
EEread_read BSF 5h,d'1' ; Data valid on rising edge of clock.
BTFSS 5h,d'0' ; Data bit into carry.
BCF status,c
BTFSC 5h,d'0'
BSF status,c
RLF temp ; Rotate carry into temp.
BCF 5h,d'1' ; Falling edge of clock.
DECFSZ clocks ; FOR I = 1 TO 8...
GOTO EEread_read
MOVF temp,w ; Move temp into EEdata
MOVWF EEdata
MOVLW d'0' ; Restore D to output.
TRIS 5h
BCF 5h,d'2' ; Deselect the EEPROM.
RETLW 0h
; Call to unprotect after EEdisbl or power up or any time EEPROM
; has been write protected.
EEnable BSF 5h,d'2' ; Select the EEPROM.
MOVLW d'12' ; Set up to shift out opcode.
MOVWF clocks
MOVLW EWEN ; temp = erase/write-enable op code.
MOVWF temp
CALL Shout ; Shift temp out to the EEPROM.
BCF 5h,d'2' ; Done. Deselect EEPROM.
RETLW 0h
; Call to protect against accidental write/erasure. Manufacturer advises
; leaving EEPROM in write-protected state most of the time to prevent accidental
; alteration by a runaway program.
EEdisbl BSF 5h,d'2' ; Select the EEPROM.
MOVLW d'12' ; Set up to shift out opcode.
MOVWF clocks
MOVLW EWDS ; temp = erase/write-disable op code.
MOVWF temp
CALL Shout ; Shift temp out to the EEPROM.
BCF 5h,d'2' ; Done. Deselect EEPROM.
RETLW 0h
; Write the byte in EEdata to EEaddr.
EEwrite MOVLW WROP ; Get the write op code.
MOVWF temp
MOVLW d'4' ; Prepare to shift out 4 msbs.
MOVWF clocks
BSF 5h,d'2' ; Select the EEPROM.
CALL Shout ; Shift temp out to the EEPROM.
MOVLW d'8' ; Now prepare for 8 addr bits.
MOVWF clocks
MOVF EEaddr,w ; Move address into temp.
MOVWF temp
CALL Shout ; Shift temp out to the EEPROM.
MOVLW d'8' ; Prepare for 8 data bits.
MOVWF clocks
MOVF EEdata,w ; Move data into temp.
MOVWF temp
CALL Shout ; Shift temp out to the EEPROM.
BCF 5h,d'2' ; Done. Deselect EEPROM.
RETLW 0h
; Erase the byte in EEaddr. Erasure leaves 0FFh (all 1s) in the byte.
EErase MOVLW ERAS ; Get the erase op code.
MOVWF temp
MOVLW d'4' ; Prepare to shift out 4 msbs.
MOVWF clocks
BSF 5h,d'2' ; Select the EEPROM.
CALL Shout ; Shift temp out to the EEPROM.
MOVLW d'8' ; Prepare for 8 addr bits.
MOVWF clocks
MOVF EEaddr,w ; Move address into temp.
MOVWF temp
CALL Shout ; Shift address out to the EEPROM.
BCF 5h,d'2' ; Done. Deselect EEPROM.
RETLW 0h
; Erase the entire EEPROM--all 256 bytes. Call before using EEwrall below.
EEwipe BSF 5h,d'2' ; Select the EEPROM.
MOVLW ERAL ; Get the erase-all opcode.
MOVWF temp
MOVLW d'12' ; Prepare to shift out 12 bits.
MOVWF clocks
CALL Shout ; Send op code + don't-care bits.
BCF 5h,d'2' ; Done. Deselect EEPROM.
RETLW 0h
; Write the byte in EEdata to every address. Call EEwipe first.
EEwrall BSF 5h,d'2' ; Select the EEPROM.
MOVLW WRAL ; Get the write-all opcode.
MOVWF temp
MOVLW d'12' ; Prepare to shift out 12 bits.
MOVWF clocks
CALL Shout ; Send op code + don't-care bits.
MOVLW d'8' ; Prepare to shift out 8 data bits.
MOVWF clocks
MOVF EEdata,w ; Move data into temp.
MOVWF temp
CALL Shout ; Shift temp out to EEPROM.
BCF 5h,d'2' ; Done. Deselect EEPROM.
RETLW 0h
; Check flag to determine whether the EEPROM has finished its self-timed
; erase/write. Caution: This will lock up your program until D goes high.
Busy MOVLW d'1' ; Set D to input.
TRIS 5h
BSF 5h,d'2' ; Select the EEPROM.
Busy_wait BTFSS 5h,d'0' ; Wait until D goes high (end of write).
GOTO Busy_wait
BCF 5h,d'2' ; Done. Deselect EEPROM.
MOVLW d'0' ; Restore D to output.
TRIS 5h
RETLW 0h
; Delay routine for demo.
Delay DECFSZ tick1
GOTO Delay
DECFSZ tick2
GOTO Delay
RETLW 0h
; EEPROM (related subroutines)
; This is a collection of subroutines for reading, writing, and
; erasing the 93LC56 (or 'C56) serial EEPROM. As a demonstration, it
; writes a scanning pattern to the 256 bytes of the EEPROM, and then
; reads it back to eight LEDs connected to port rb.
device pic16c54,xt_osc,wdt_off,protect_off
reset start
D = ra.0 ; Pins DI and DO of the EEPROM
CLK = ra.1 ; Clock pin--data valid on rising edge
CS = ra.2 ; Chip select--high = active
ROP = 192 ; Opcode for read
WROP = 160 ; Opcode for write
EWEN = 152 ; Opcode to enable erase and write
EWDS = 128 ; Opcode to disable erase and write
ERAS = 224 ; Opcode to erase a byte
ERAL = 144 ; Opcode to erase all bytes
WRAL = 136 ; Opcode to write all bytes
org 8
temp ds 1 ; Temporary variable for EE routines
EEdata ds 1 ; Data to EEwrite/wrall, from EEread
EEaddr ds 1 ; Address to EErase, EEwrite, EEread
clocks ds 1 ; Number of clock cycles for SHiftOUT
tick1 ds 1 ; Timer for demo.
tick2 ds 1 ; Timer for demo.
org 0
start mov ra,#0 ; Clear ports
mov rb,#0
mov !ra,#0 ; Make all ra pins outputs initially.
mov !rb,#0 ; Output for LEDs.
call EEnable ; Turn off write/erase protection.
mov EEdata,#1 ; EEdata = 1.
mov EEaddr,#0 ; Start at address 0.
:loop call EEwrite ; Write pattern to EEPROM
call Busy ; Wait until done writing.
rr EEdata ; Rotate the pattern right.
ijnz EEaddr,:loop ; FOR EEaddr = 0 to 255...
call EEdisbl ; Turn write/erase protection back on.
:loop2 call EEread ; Now read back contents of EEPROM.
mov rb,EEdata ; Display the EEPROM data on LEDs.
inc EEaddr ; Set up for next address.
call delay ; Delay to slow down the scanning.
goto :loop2 ; Do forever.
; Shout shifts out the bits of the variable temp to the EEPROM data line.
; All of the other EEPROM routines call Shout.
Shout rl temp ; Rotate bit7 of temp into carry
movb D,c ; Move carry bit to input of EEPROM
setb CLK ; Clock the bit into EEPROM
nop ; Clock must be high > 500 ns
clrb CLK
djnz clocks,Shout ; FOR I = 1 TO clocks...
ret
; Read the byte in EEaddr into EEdata.
EEread mov temp,#ROP ; Move the read opcode into temp
mov clocks,#4 ; Number of bits to shift out (op+1)
setb CS ; Select the EEPROM.
call Shout ; Shift the read opcode out.
mov clocks,#8 ; Number of bits to output (8 addr bits).
mov temp,EEaddr ; Set up to send the address.
call Shout ; Shift out the address.
mov !ra,#1 ; Now, change D to input.
mov clocks,#8 ; Number of bits to input (8 data bits).
:read setb CLK ; Data valid on rising edge of clock.
movb c,D ; Data bit into carry.
rl temp ; Rotate carry into temp.
clrb CLK ; Falling edge of clock.
djnz clocks,:read ; FOR I = 1 TO 8...
mov EEdata,temp ; Move temp into EEdata
mov !ra,#0 ; Restore D to output.
clrb CS ; Deselect the EEPROM.
ret
; Call to unprotect after EEdisbl or power up or any time EEPROM
; has been write protected.
EEnable setb CS ; Select the EEPROM.
mov clocks,#12 ; Set up to shift out opcode.
mov temp,#EWEN ; temp = erase/write-enable op code.
call Shout ; Shift temp out to the EEPROM.
clrb CS ; Done. Deselect EEPROM.
ret
; Call to protect against accidental write/erasure. Manufacturer advises
; leaving EEPROM in write-protected state most of the time to prevent accidental
; alteration by a runaway program.
EEdisbl setb CS ; Select the EEPROM.
mov clocks,#12 ; Set up to shift out opcode.
mov temp,#EWDS ; temp = erase/write-disable op code.
call Shout ; Shift temp out to the EEPROM.
clrb CS ; Done. Deselect EEPROM.
ret
; Write the byte in EEdata to EEaddr.
EEwrite mov temp,#WROP ; Get the write op code.
mov clocks,#4 ; Prepare to shift out 4 msbs.
setb CS ; Select the EEPROM.
call Shout ; Shift temp out to the EEPROM.
mov clocks,#8 ; Now prepare for 8 addr bits.
mov temp,EEaddr ; Move address into temp.
call Shout ; Shift temp out to the EEPROM.
mov clocks,#8 ; Prepare for 8 data bits.
mov temp,EEdata ; Move data into temp.
call Shout ; Shift temp out to the EEPROM.
clrb CS ; Done. Deselect EEPROM.
ret
; Erase the byte in EEaddr. Erasure leaves 0FFh (all 1s) in the byte.
EErase mov temp,#ERAS ; Get the erase op code.
mov clocks,#4 ; Prepare to shift out 4 msbs.
setb CS ; Select the EEPROM.
call Shout ; Shift temp out to the EEPROM.
mov clocks,#8 ; Prepare for 8 addr bits.
mov temp,EEaddr ; Move address into temp.
call Shout ; Shift address out to the EEPROM.
clrb CS ; Done. Deselect EEPROM.
ret
; Erase the entire EEPROM--all 256 bytes. Call before using EEwrall below.
EEwipe setb CS ; Select the EEPROM.
mov temp,#ERAL ; Get the erase-all opcode.
mov clocks,#12 ; Prepare to shift out 12 bits.
call Shout ; Send op code + don't-care bits.
clrb CS ; Done. Deselect EEPROM.
ret
; Write the byte in EEdata to every address. Call EEwipe first.
EEwrall setb CS ; Select the EEPROM.
mov temp,#WRAL ; Get the write-all opcode.
mov clocks,#12 ; Prepare to shift out 12 bits.
call Shout ; Send op code + don't-care bits.
mov clocks,#8 ; Prepare to shift out 8 data bits.
mov temp,EEdata ; Move data into temp.
call Shout ; Shift temp out to EEPROM.
clrb CS ; Done. Deselect EEPROM.
ret
; Check flag to determine whether the EEPROM has finished its self-timed
; erase/write. Caution: This will lock up your program until D goes high.
Busy mov !ra,#1 ; Set D to input.
setb CS ; Select the EEPROM.
:wait jnb D,:wait ; Wait until D goes high (end of write).
clrb CS ; Done. Deselect EEPROM.
mov !ra,#0 ; Restore D to output.
ret
; Delay routine for demo.
Delay djnz tick1,Delay
djnz tick2,Delay
ret
See also:
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