Timing of writing PHSx

Andrew E Mileski

Sorry if this is noted elsewhere, but I couldn't find anything via Google using site:forums.parallax.com

I'm trying to output 2 cycle long bit periods back-to-back using multiple counters on multiple synchronized cogs.

As I understand it, writes to PHSx in instructions as dest actually write to shadow memory first which is then latched into the counter.

Q1: If a counter is running, does the first potential accumulate after writing PHSx happen on clock cycle M + 5 (stage 5) or M + 6 (the next stage 1)?

Q2: In NCO / PWM modes, can PINA change immediately upon writing of PHSx[noparse][[/noparse]31] on clock cycle M + 5, or does an accumulate need to happen first?

Thanks.

Phil Pilgrim (PhiPi)

Andrew,

It looks like PHSA and any resulting pin changes are both written "almost immediately" during the R cycle. Here's a program I wrote to test this:

[b]CON[/b]

  [b]_clkmode[/b]      = [b]xtal1[/b] + [b]pll16x[/b]
  [b]_xinfreq[/b]      = 5_000_000
  
  pin0          = 5
  pin1          = 6
  
[b]PUB[/b]  Start

  [b]cognew[/b](@go, 0)

[b]DAT[/b]

go      [b]mov[/b]   [b]dira[/b],[b]dira[/b]0
        [b]mov[/b]   [b]ctra[/b],[b]ctra[/b]0
        [b]mov[/b]   [b]frqa[/b],#1         

:golp   [b]or[/b]    [b]outa[/b],[b]outa[/b]0
        [b]mov[/b]   [b]phsa[/b],[b]phsa[/b]0
        [b]andn[/b]  [b]outa[/b],[b]outa[/b]0
        [b]mov[/b]   [b]phsa[/b],#0
        [b]jmp[/b]   #:golp

[b]dira[/b]0   [b]long[/b]  1 << pin0 | 1 << pin1
[b]outa[/b]0   [b]long[/b]  1 << pin0
[b]ctra[/b]0   [b]long[/b]  %00100 << 26 | pin1
[b]phsa[/b]0   [b]long[/b]  $8000_0000

Here's the resulting scope trace:



The delay from writing the first pin directly to writing the second one indirectly via PHSA is 52 ns, or 2 ns (a fraction of a cycle) longer than one instruction time. I repeated the experiment by just writing both pins directly in the same sequence. The delay was 50 ns. So the extra 2 ns definitely comes from writing PHSA and the result percolating to the pin.

-Phil

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'Just a few PropSTICK Kit bare PCBs left!

Andrew E Mileski

Thanks Phil! Much appreciated.