timer0 (18F452, MIOS 1.9f, asm)

MIOS programming (Assembler)
1

hi,

my aim is to create a sort of Fx function for the MIDIBox AY application - following the conception of sound tracker.

the USER_Timer is in use by the SM_DebounceTimer (c64 keyboard / 8x8 sm driver).

so i thought it might be a good idea to work with the timer0

i’m new to timer and interrupt programming. i read some lines about the basic concept of the timer in general but i don’t know where to start in the context of MIOS.

basic concept of the Fx function:

[midi event note on] ==> toggle timer0 on

after timer0 cycle 1) Fx 01 write x to reg a

after timer0 cycle 2) Fx 02 write y to reg a

after timer0 cycle 3) Fx 03 write z to reg a

after timer0 cycle 4).Fx 04 write w to reg c

etc.

[midi event note off] ==> toggle timer0 off

so now my question:

  1. how to initialize and activate timer0?

  2. what is to consider in relation to interrupts?

  3. is there already some code with timer0 in action? (where i can learn from)

greetings,

  • lemonhorse
2

Hi,

the PIC based MIOS doesn’t allow you to add interrupt handlers (this has changed in MIOS32)

This means, that the timer overrun flag has to be polled by a routine which is executed periodically (e.g. from the USER_Timer hook, if it is called more often than the target interval of TMR0)

Under http://svnmios.midibox.org/filedetails.php?repname=svn.mios&path=%2Ftrunk%2Fapps%2Fsynthesizers%2Fmidibox_sid_v2%2Fsrc%2Fsid_se.inc

You will find a very simple example - the BPM generator is executed from USER_Timer each mS, but it uses it’s own time base TMR0

        ;; check timer0 overrun flag
        BRA_IFCLR INTCON, TMR0IF, ACCESS, SIDSE_Clk_NoInc
        bcf     INTCON, TMR0IF                  ; clear overrun flag
        SET_BSR TIMER0_RELOAD_L
        bcf     T0CON, TMR0ON
        movf    TIMER0_RELOAD_L, W, BANKED      ; (dummy add to update TMR0H, and to get carry flag of addition)
        addwf   TMR0L, W                        ; (update TMR0H)
        movf    TIMER0_RELOAD_H, W, BANKED
        addwfc  TMR0H, F
        movf    TIMER0_RELOAD_L, W, BANKED
        addwf   TMR0L, F                        ; 16bit register update takes place with this write
        bsf     T0CON, TMR0ON
        SET_BSR SID_BASE
SIDSE_Clk_Inc
... your code which will be executed periodically
SIDSE_Clk_NoInc
[/code]


TIMER0_RELOAD_[LH] defines the timer interval: 0x10000 - (interval / (2*100 nS))
e.g., a reload value of 0xc000 leads to an interval of 3.277 mS

Initialisation in USER_Init:
[code]  
       ;; ensure that timer0 interrupt not enabled  
        bcf     INTCON, T0IE  
  
        ;; internal clock source, 16bit, prescaler 1:2  
&nbsp; &nbsp; &nbsp; &nbsp; movlw&nbsp; &nbsp;(1 \<\< TMR0ON)  
&nbsp; &nbsp; &nbsp; &nbsp; movwf&nbsp; &nbsp;T0CON

Best Regards, Thorsten.

3

thanks TK,

now i got stuff for experimentation!  :slight_smile: