Re: Hey, what is all this 'off topic' posting?

On Sun, 04 Sep 2005 23:19:39 GMT, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

>Hello John,
>
>>>>I've been grinding out assembly code all week. There's nothing much to
>>>>say about that.
>
>That made me chuckle. Thinking about Forrest Gump ("That's all I have to
>say about that"). SCNR.
>
>>>Might I ask why you're using assembler
>>
>> Because it's fast, because I know exactly what's going on, and because
>> I like it.
>
>That's also why I often design stuff discrete. It takes out the not so
>documented habits of some chips, IOW the ones that marketing didn't like
>to see on the data sheet because it wouldn't look good. Like that LDO
>regulator that didn't like it when the source resistance got too high.
>
>> ... The customer called Friday and told me he would buy a bunch
>> more if only I can make it four times as fast as we'd planned for.
>
>I always thought we'd be immune from that once we start our own
>business. About 15 years ago I realized that wasn't so. "Oh, can you
>make those two functions run simultaneously?". Take deep breaths, open
>the window no matter how cold it is, don't let the face get too red,
>bite on the lips so no profanities come out and all that.

It's a 16-channel isolated DAC module, mainly intended to simulate
thermocouples, but capable of up to +-12.5 volts out. We used a 16-MHz
CPU running code out of a single 8-bit EPROM because we figured, how
fast can a temperature change? So now the customer wants it to operate
in voltage mode, fast.

So R. claims he can take over the DAC calibrations (one 16-bit signed
fractional multiply and one add, per channel) and the dac-load state
machine in the FPGA to save me lots of microseconds per channel and,
sure as hell, it works. He even claims 1/8 LSB rounding accuracy. The
18x18 multiplier in the FPGA has a prop delay of 6 ns!

Next, after tweaking the code as much as is decent, I'm going to copy
the critical loops from eprom into fast CPU-internal 16-bit RAM, and
run it there. It's easy to write pic (position-independent code) on a
68K.

I've asked a couple guys if it's feasible to relocate C-compiled code
like this, and they say it depends on the compiler but it's not really
obvious. In assembly, it's easy.

John


.

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