Thanks for the commentary. For the sake of argument, I'll respond, heck,
maybe someone will see something that I missed.
:> I have a pullup and a cap AND a DS1233 supervisor on the reset line
: I tool a look at the MEGA8535 specs and have some questions for you. (note: I
: haven't dug through any errata)
: What is your VCC voltage?
: What brownout detection setting are you using? What does your power supply
: look like?
The supervisor, I've turned BOD off in the chip.
: Have you tried taking a snapshot of the MCU Control and Status register when a
: reset occurs (latch its contents out to I/O pins, or if you have a debugger,
: breakpoint after a reset and look at it, etc.). It might help narrow down on
: the source of your reset.
I've looked, but for the life of me I don't find a register that tells
me that I just had a reset - or for what purpose it reset. The PIC has
me spoiled there, it DOES have a register that tells that. I suppose that
I could just spit something out at the beginning of the program, assuming
a reset, I'll hold that thought in reserve, it is a bit of a pain to do
: Table 15 and Figures 166-168 and 194 are interesting.
Table 15 was my first study. However, my MEGA8535 doc stops at figure
130, where did you get yours?
: Some microcontrollers will spec the maximum rise and fall times of the reset
: line. Atmel doesn't, so it's hard to say what it expects (it's worth a phone
: call to an application engineer). The DS1233 has a 5K pullup. What size cap
: have you added? What size external pullup are you using? If you're
: transitioning too slow you may have some strange results (i.e. different
: sections of the micro may or may not reset, or may not completely reset).
10K, .1uf, well in the spec for the chip, which is a ds1233-10, so 4.25 to
about 4.5V is the trip point. I've put a scope on the 5V line and it is as
clean as driven snow, no noise there at all.
: Another spec that's commonly given is if the micro needs a certain number of
: clock cycles before reset goes inactive. The Atmel spec states that external
: reset will work even in the absence of a clock. They do have some interesting
: power up delay settings for different oscillator configurations and power up
: scenarios. What are you using for the settings? What is your oscillator
Slow power rampup, external resonator. Using a 16MHz resonator.
: With the built in power on reset and brownout detection, do you really need an
: external reset controller? Could you temporarily disconnect the reset line
: altogether (cut the trace), eliminating the question of crosstalk?
I found that it did not properly reset the board on power up, that is the
reason that I added the supervisor. I'll try the trace cut again, but
since I use the reset line for the ICSP, there is a limit to what I can
cut out. There is a possibility (potentially high) that the two issues
are related. As you noted, Atmel doesn't GIVE a spec on the reset/power
issue so deciding what to use becomes a matter of picking what worked
: Sometimes the simplest problems are the hardest to debug...
No doubt. I may end up taking this one back "to the board" for a
re-evaluation of the PCB design.
* Dennis Clark firstname.lastname@example.org www.techtoystoday.com *
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