Lpc2000

---- Original Message ----
From: "brendanmurphy37" <brendan.murphy@...>
To: <lpc2000@yahoogroups.com>
Sent: Friday, January 20, 2006 4:38 PM
Subject: [lpc2000] Re: D/A noise

> Wow! that's getting really esoteric!
>
> Magnus: Karl is right about the jitter. Even without the delay
> introduced by the timer register being on the peripheral bus, you
> have to take into acount the timer could tick either imediately after
> you load it or any time from then until you load it again. Even if
> the number of instructions and clocks in this is always the same, the
> tick could happen anywhere in the sequence.

You should have no pclk or timer prescaler so that the timer ticks at the
cclk rate.  So during a single load from T0TC, it actually increments eight
times.  Exactly in which of the eight clocks the T0TC value is latched
doesn't really matter jitterwise, as long as it is the same every time.

I just tried this program on my LPC2103:

/* Vector area relocated to RAM */
/* r8_fiq preloaded with T0 = 0xE0004000 */
/* r9_fiq preloaded with address of ram_fiqhandler */

/* 0x001C: */

/* Timer 0 has just generated an interrupt and been reset.  T0TC is now
 * a small positive number that tells how many clocks it took to enter the
 * FIQ handler. */
ldr  r10, [r8, #+8]   /* r10 = initial T0TC */

/* If it is not unreasonably large (indicating an unexpected big latency,
 * such as if somebody has had FIQs disabled), jump into ram_fiqhandler,
 * skipping as many nops as we have already spent clocks. */
cmp  r10, #32
addls pc, r9, r10, lsl #2

/* It was unreasonably large, jitter not completely eliminated.
 * Just skip all the nops. */
add  pc, r9, #4*32


ram_fiqhandler:    /* located in RAM */
nop
nop
... total 32 nops
nop
ldr  r11, [r8, #+8]   /* r11 = final T0TC */
...

At this time, r10 is the initial T0TC value, and r11 is the T0TC value after
jitter elimination.

The program showed that the initial T0TC value varied between 9 and 16, and
the final T0TC was always 44.  So the normal FIQ jitter of 7 clocks (which
could be larger if the foreground contained things like big ldm/stm's) was
reduced to a jitter of 0.

I think this method is simpler and wastes fewer clocks than multi-stage
interrupt handlers.  But I still think that Philips should have added some
hardware synchronisation to the DAC.

Karl Olsen