;***********************************************************************
;* This program demonstrates a glitch in the MSP430F2013's USI module. *
;* I've tested it successfully with the EZ430-F2013, as well as other  *
;* setups.  If the glitch doesn't show up within a few moments, try    *
;* modifying the impedance of SCL very slightly by placing your hand   *
;* near, or attaching a long (eg, 24 inches) conductor to pin 8.       *
;***********************************************************************
#include  "msp430x20x2.h"

            RSEG    CSTACK                  ; Define stack segment
            RSEG    CODE                    ; Assemble to Flash memory
            EVEN

RESET       mov #0x5A80, &0x0120      ; Stop watchdog timer
            mov.w #SFE(CSTACK),SP     ; Initialize the stack pointer

            ; Set the clock speed to around 1MHz
            mov #0x86BA, &0x0056

            ; Setup pin assignments
            mov.b #0xFF, &0x0021      ; P1OUT = 0xFF
            mov.b #0xFF, &0x0022      ; P1DIR = 0xFF
            mov.b #0x00, &0x0026      ; P1SEL = 0x00
            mov.b #0xC0, &0x0027      ; P1REN = 0xC0  ; P1.6 & P1.7 Pullups

            ; Setup the USI in master mode
            mov.b #0xCB, &0x0078      ; USICTL0    ; Initiate software reset
            mov.b #0x50, &0x0079      ; USICTL1
            mov.b #0x6A, &0x007A      ; USICKCTL   ; SMCLK / 8
            mov.b #0x00, &0x007B      ; USICNT = USIIFGCC
            mov.b #0xCA, &0x0078      ; Finish the software reset
            mov.b #0x50, &0x0079      ; Clear the interrupt flags

            eint

main_loop
            ; Begin with a short delay
            mov #500, r12
delay
            dec r12
            jne delay 

            clr r7
            mov.b #0x08, &0x007B      ; Start transmitting 8 bits

;****************************************************************
;* We watch for the glitch while the byte is being transmitted. *
;* The symptom is that USICNT spontaneously takes on a value    *
;* higher than what was assigned to it.  A typical observed     *
;* value is around 0x1A.                                        *
;****************************************************************
watch_for_glitch
            cmp.b #0x09, &0x007B
            jhs glitch
            tst r7
            jeq watch_for_glitch
            jmp main_loop

glitch
;************************************************************************
;* Place a breakpoint here or trigger your oscilloscope on P1.0 (pin 2) *
;************************************************************************
            dint
            bic.b #1, &0x0021
            bis.b #1, &0x0021
            mov.b #0x00, &0x007B
            bic.b #3, &0x0079
            eint
            jmp main_loop

;*****************************
;* Interrupt Service Routine *
;*****************************
USI_ISR     mov #1, r7
            bic.b #1, &0x0079
            reti

            COMMON  INTVEC
            ORG     RESET_VECTOR            ; MSP430 RESET Vector
            DW      RESET                   ;
            ORG     USI_VECTOR              ; USICNT
            DW      USI_ISR                 ;
            END