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Another feature of PBASIC's Serin is the ability to compare incoming serial data to a table of qualifiers. Serin halts program execution until it receives data that exactly matches the qualifiers. This makes it possible to recognize key words in a text stream, or to protect access to a sensitive program.
You can add this capability to Serin using the routine Qualify and a few additional instructions. The listing shows how this works. Rather than present the entire Serin listing again, only the Qualify code appears here. To simulate receiving data with Serin, execute up to the ret at the end of the dummy Serin, then enter a byte at address 01Eh (buffer-1).
Qualifiers must match the case of the password. "SESAME" is not the same as "Sesame" as far as Qualify is concerned. Also, make sure to turn off the Serin filter flag if your password contains characters outside the range of filt_lo to filt_hi.
Serin and Qualify will wait forever to receive the password sequence. You may add code to count the number of bytes received and give up if the proper sequence is not submitted within a preset number of tries.
; ; *************************************************************************** ; *** Bubble Software Parallax to PIC Source Converter. Copyright 1999. *** ; *** http://www.picnpoke.com email: sales@picnpoke.com *** ; *************************************************************************** ; ; QUALIFY table, string ; Reads a one-character Serin string and looks up the character in ; a table of "qualifiers." If the character matches the next qualifier, ; the routine increments a counter and checks to determine whether all ; qualifiers have been matched. If so, it returns a 1 in w. If not, ; it returns a 0 in w after resetting the counter to the first item in ; the qualifier list. ; Device data and reset vector P = pic16c55 #include <16c55.inc> ; processor assembler definitions _CONFIG _xt_osc & _wdt_off & _protect_off reset start org 8 qual Res d'1' ; Pointer to qualifiers. buffer equ d'31' ; Serin string address. org 0 P_word ADDWF pcl RETLW d'6' ; No. of elements in password. RETLW 'S' ; The password. RETLW 'e' RETLW 's' RETLW 'a' RETLW 'm' RETLW 'e' ; Below is a dummy Serin routine intended for use on the PSIM simulator. ; Wait until PSIM reaches the "ret" below, then change the data at buffer-1 ; (hex address 1E). You can step through Qualify to see how it works. ; The password "Sesame" is 53h, 65h, 73h, 61h, 6Dh, 65h. Serin MOVLW 'A' ; Dummy Serin for demo. MOVWF buffer-1 RETLW 0h ; Enter character data here. start MOVLW d'4' ; Input for serial data. TRIS 5h MOVLW d'0' ; All outputs for LEDs TRIS 6h CLRF 6h ; LEDs off to begin. CLRF qual ; Clear qual to start. start_loop CALL Serin ; Receive one byte. CALL Qualify ; Check it against password. IORLW 0h ; IF w=1 THEN LEDs_on BTFSC status,z ; ELSE :loop GOTO start_loop MOVLW d'255' ; Password match: toggle LEDs. XORWF 6h GOTO start ; Do it again. ; Upon entry, a one-byte Serin string should be stored at buffer-1. ; Qualify will check it against the next entry in the P_word table, and ; if there's a match, will increment the qualifier counter qual. If there's ; no match, it will clear qual, and check the character against the first ; entry in P_word. This second step prevents Qualify from failing to ; recognize passwords after a false start, like "SeSesame." Qualify INCF qual,w ; Look up next item in P_word. CALL P_word subwf buffer-1,0 ; Compare it to received byte by <Microchip instruction> BTFSS status,z ; subtraction. If result <> 0, no match. GOTO Qualify_no_match INCF qual ; Match: increment qual pointer and CLRW ; get the 0th item in the table (number CALL P_word ; of chars in the password). subwf qual,0 ; IF qual = no._of_P_word_chars THEN <Microchip instruction> BTFSC status,z ; return with 1 in w. RETLW d'1' ; ELSE return with 0 in w. RETLW 0h Qualify_no_match MOVF qual ; IF qual = 0, return with 0 in w. BTFSC status,z RETLW 0h CLRF qual ; ELSE qual = 0: GOTO Qualify (check GOTO Qualify ; char against 0th password entry). end
; QUALIFY table, string ; Reads a one-character Serin string and looks up the character in ; a table of "qualifiers." If the character matches the next qualifier, ; the routine increments a counter and checks to determine whether all ; qualifiers have been matched. If so, it returns a 1 in w. If not, ; it returns a 0 in w after resetting the counter to the first item in ; the qualifier list. org 8 qual ds 1 ; Pointer to qualifiers. buffer = 31 ; Serin string address. ; Device data and reset vector device pic16c55,xt_osc,wdt_off,protect_off reset start org 0 P_word jmp pc+w retw 6 ; No. of elements in password. retw 'Sesame' ; The password. ; Below is a dummy Serin routine intended for use on the PSIM simulator. ; Wait until PSIM reaches the "ret" below, then change the data at buffer-1 ; (hex address 1E). You can step through Qualify to see how it works. ; The password "Sesame" is 53h, 65h, 73h, 61h, 6Dh, 65h. Serin mov buffer-1,#'A' ; Dummy Serin for demo. ret ; Enter character data here. start mov !ra,#4 ; Input for serial data. mov !rb, #0 ; All outputs for LEDs clr rb ; LEDs off to begin. clr qual ; Clear qual to start. :loop call Serin ; Receive one byte. call Qualify ; Check it against password. test w ; IF w=1 THEN LEDs_on snz ; ELSE :loop jmp :loop XOR rb,#255 ; Password match: toggle LEDs. jmp start ; Do it again. ; Upon entry, a one-byte Serin string should be stored at buffer-1. ; Qualify will check it against the next entry in the P_word table, and ; if there's a match, will increment the qualifier counter qual. If there's ; no match, it will clear qual, and check the character against the first ; entry in P_word. This second step prevents Qualify from failing to ; recognize passwords after a false start, like "SeSesame." Qualify mov w,++qual ; Look up next item in P_word. call P_word subwf buffer-1,0 ; Compare it to received byte by jnz :no_match ; subtraction. If result <> 0, no match. inc qual ; Match: increment qual pointer and clr w ; get the 0th item in the table (number call P_word ; of chars in the password). subwf qual,0 ; IF qual = no._of_P_word_chars THEN snz ; return with 1 in w. retw 1 ; ELSE return with 0 in w. ret :no_match test qual ; IF qual = 0, return with 0 in w. snz ret clr qual ; ELSE qual = 0: GOTO Qualify (check jmp Qualify ; char against 0th password entry).See also:
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