MSP430 LPM4, UART wakeup, crashing...

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Hi all, I'm currently working in an application that requires the MCU to g
into deep sleep (LPM4) and wake up and process data on UART receive, an
then go back to LPM4. What is the right way to do this? I tried doing th
following as TI's guide Rev. E. suggests, but this crashes the MCU.
tried to debug the code but I just can't find where the problem is. Th
serial data is received using IR, in a constant stream that does not sto
until the "transmitter" is taken away from the receiver. This will wor
fine for a while but it eventually crashes.

What I've seen is that somehow when the software is busy handling a
interrupt, it STILL accepts other interrupts even when the GIE bit is off
Sometimes the instruction that sends the MCU back to active mode doesn't d
anything or messes everything up. I really need help here as I'm runnin
out of ideas. BTW, the problem is not on hardware, all of the require
pins are correctly terminated to either VCC or ground, even th
programming pins, reset, etc.

#include "msp430x12x.h"
          ORG 0FFFEh               ; power-up interrupt
          DW RESET

          ORG 0FFEEh               ; USART0 receive interrupt
          DW RECEIVE

          ORG 0F000h               ; program start for MSP430F122
; Reset Interrupt sequence
          MOV.W #0300h, SP         ; initialize stack pointer
          CALL #INIT               ; initalize the UART

          BIS #LPM4+GIE, SR        ; wait for start-edge
      NOP                   ; Needed by debugger
      CMP.B #f0, r8            ; check received data
          JNZ MAIN                 ; incorrect data! jmp!
          XOR.B #1, P1OUT          ; toggle LED pin to ack received flag
          JMP MAIN

; USART Interrupt sequence
          BIT.B #URXIFG0,&IFG2      ; Test URXIFGx to determine
          JNZ StartRec              ; If start-edge or character
                                    ; if IFG is set, Z=0, continue.
                                    ; if IFG not set (no complet
transmission), return!

StartEdge                           ; wake up and receive char
          BIC.B #URXSE,&U0TCTL      ; Clear URXS signal
          BIS.B #URXSE,&U0TCTL      ; Re-enable edge detect
          BIC #SCG0+SCG1,0(SP)      ; Enable BRCLK = DCO; CPU still off
          JMP EndRcv

          MOV.B &U0RXBUF, r8        ; Save received char in r8
      BIC #OE+FE+PE+RXERR,U0RCTL ;Clear any errors received
          BIC #LPM4+GIE,0(SP)       ; Char received, wake up and proces


; Initialization function (sets up UART, flags and interrupts)
          MOV.W #WDTPW+WDTHOLD, &WDTCTL   ; stop watchdog
          BIS #OSCOFF, SR                 ; not using LFXTL1

          //Initialize UART
          BIS.B #CHAR, &U0CTL
                                  ; 1 stop bit
                                  ; 8-bit word
          BIS.B #SSEL1, &U0TCTL   ; use sub-main clock for UART (@750kHz)
          MOV.B #RATIO, &U0BR0    ; f = 750kHz in MSP430F122
          MOV.B #000h, &U0BR1     ;
          MOV.B #00h, &U0MCTL     ; receive moodulation set to zero; 0.0
is small
          BIS.B #URXSE, &U0TCTL   ; enable start-edge wake-up
          BIS.B #020h, &P3SEL     ; select P3.5 for serial reception
          BIC.B #020h, &P3DIR     ; and input
          BIS.B #URXE0, &ME2      ; enable receiver (MSP430F122)
          BIC.B #SWRST, &U0CTL    ; clear UART software reset
          BIS.B #URXIE0, &IE2     ; enable receiver interrupt

      BIS.B #003h, &P1DIR      ; P1.1 and P1.0 output
          BIS.B #003h, &P1OUT     ; turn LEDs off

          EINT                    ; enable interrupts: GIE<-1



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Re: MSP430 LPM4, UART wakeup, crashing...
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IIRC,  the LPM4 mode shuts off all the clocks, including that to the
UART.   Are you sure that the UART clock is starting up quickly enough
to capture the start bit?

If the data is really received in a constant stream,  you might be
better off not going into LPM4 mode after each character---but
only when no character has been received for a pre-determined

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Mark Borgerson

Re: MSP430 LPM4, UART wakeup, crashing...
Mark is right.  Been there, done that.

When you sleep in LPM4, there is only one way to get out of it: you can
use an interrupt from any peripheral that *doesn't* use a clock.  All
clocks are off in LPM4.

Waking from LPM4 is slow because it takes some time for the clocks to
restart.  If you're app demands single clock cycle response on wakeup,
then LPM3 might be a better choice for sleeping.

They call LMP4 "done mode".  Please take that tag literally.  You're
app woll be done doing whatever it's doing if it's in LPM4.  LPM is
called "sleep mode".  Your app will be active but waiting to wakeup
(IOW, not done).

There is a useful table in the User' Guide that shows which clocks are
on and off for the various LPMs.


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Re: MSP430 LPM4, UART wakeup, crashing...
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Just some thoughts:
- Does your code work properly while not going LPM or going to another LPM?

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- You turn it on inside the interrupt routine. Don't do it, the CPU will
turn it on while executing RETI.

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You don't properly switch the CPU to inactive mode - you don't use
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GIE is already 1 before this instruction.

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Needed only if there is nothing after it - it's not your case.

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Why do you turn interrupts off here?
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Actually it's an output here.

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