How can I speed this code up? SHIFTOUT is slowing it down and I need a faster way.

[Bruce]

Here's a neat trick to get 1MHz with a 20MHz oscillator. This takes 5 instruction cycles to toggle the pin, and Timer1 keeps track of the toggle count for you.

You don't need to use any incrementing or decremeting loops or variables.

1. Set T1CKI pin, and make the pin an output.
2. Load Timer1 low & high registers with 65,536 - the number of clocks to output & count.
3. Setup Timer1 for external clock, 1:1 prescaler, and turn it on.

T1CKI outputs your clock while Timer1 count increments on every low-to-high transition on T1CKI.

PORTC.0 = 1       ' set pin so 1st low-to-high increments count
TRISC.0 = 0       ' make pin an output
 
Main:
  TMR1H = $F0    ' 65,536 - 4096 = 61,440 = $F000
  TMR1L = $00    ' so 4096 toggles will set TMR1IF
  T1CON = %00000011 ' 1:1 prescaler, external clock, Timer1 on
 
ASM
Pulse
    BCF PORTC,0       ; clear T1CKI pin
    BSF PORTC,0       ; set T1CKI pin
    BTFSS PIR1,TMR1IF ; when TMR1 overflows, count is complete
    GOTO Pulse        ; loop until Timer1 overflow
    BCF  PIR1,TMR1IF  ; clear TMR1 overflow flag bit
ENDASM

When PIR1,TMR1IF = 1 you have 4096 clocks. I tested this on a 16F877A, so just change the T1CKI pin to whatever it is on your PIC type.

[Mike, K8LH]

Son of a gun, that works (please note that T0CKI is RB6 on a 16F88).

The three cycle loop 'overhead' (BTFSS & GOTO) is a bottleneck. You could get better performance if you spread it out over more pulses. If you were to produce 16 pulses within the loop then you could use a single byte variable counter instead of Timer 1 and bump the output up to 2+ MHz.

Regards, Mike

[wolwil]

Thank You Bruce for the code!!! I will try to get it loaded and tested tonight.

Son of a gun, that works (please note that T0CKI is RB6 on a 16F88).

The three cycle loop 'overhead' (BTFSS & GOTO) is a bottleneck. You could get better performance if you spread it out over more pulses. If you were to produce 16 pulses within the loop then you could use a single byte variable counter instead of Timer 1 and bump the output up to 2+ MHz.

Regards, Mike

Mike are you saying Use a FOR Loop ouside of the ASM like this or is there an ASM FOR Loop I should be using for it (Keep in mind I dont know ASM):

PORTB.6 = 1       ' set pin so 1st low-to-high increments count
TRISB.6 = 0       ' make pin an output
T1CON = %00000011 ' 1:1 prescaler, external clock, Timer1 on
C1 VAR BYTE
 
FOR C1 = 0 to 255
ASM
Pulse
    BCF PORTB,6       ; clear T1CKI pin / 1
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 2
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 3
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 4
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 5
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 6
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 7
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 8
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 9
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 10
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 11
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 12
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 13
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 14
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 15
    BSF PORTB,6       ; set T1CKI pin
    BCF PORTB,6       ; clear T1CKI pin / 16
    BSF PORTB,6       ; set T1CKI pin
ENDASM
NEXT

[Bruce]

I think this is what Mike was talking about?

This will give you 2.5MHz with every 16th logic 1 bit stretched to 4 cycles VS 1. If you can live with this bit being a tad longer, it will definitely speed things up.

 
' define port & pin use in _Pulse
@ #DEFINE PORT PORTB ' use any port you prefer, but declare it here
@ #DEFINE PIN 6      ' same as above
 
PORTB.6 = 1       ' initialize pin to idle state
TRISB.6 = 0       ' make the pin an output
 
C1 VAR BYTE BANK0 SYSTEM
 
Main:
    C1 = 0        ' clear loop count
    CALL Pulse    ' generate 4096 pulses
    GOTO Main
 
ASM
_Pulse
    BCF PORT,PIN     ; clear pin / 1
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 2
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 3
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 4
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 5
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 6
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 7
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 8
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 9
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 10
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 11
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 12
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 13
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 14
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 15
    BSF PORT,PIN     ; set pin
    BCF PORT,PIN     ; clear pin / 16
    BSF PORT,PIN     ; set pin
    DECFSZ C1,F      ; decrement count, skip if 0
    GOTO _Pulse      ; not done, keep going
    RETURN
ENDASM

With either version, make sure you have WDT disabled.

[wolwil]

I didn't have to turn off the WDT, I think its off by default but could not find where it tells me in the manual. What will happen if its on? Would I notice it?

I do have one issue but it might just be an issue with it being on a bread board but when I use the Define OSC 20 with the 20 MHz clock it freaks out and does what ever it feels like and is sensitive to the touch but when I use no Define or Define OSC 4MHz but run it with the 20MHz clock it runs fine...I don't get it haha


Thanks again everyone, I am very grateful for all your help and suggestions