AVR-Chat

At 11:53 PM 2/2/2004 -0500, Brian Dean wrote:
>After hooking up a scope and taking some measurements, I think the
>problem might be that the hardware SPI on the ATmega128 is too fast
>for the device I'm hooking up.  At the largest prescale for the
>hardware SPI, I measure the SPI clock at around 116 KHz.  This is
>close to what the datasheet says which at my crystal freq of 16 MHz,
>SPI clock should be fosc/128 = 125 KHz which is in the ballpark.

<VBG>
My current project is running 4 MHz SPI, and at that, I have two pushpops 
waiting for the byte to clear. :)
If I could get the SPI on the M8, with internal RC osc, up to 6 MHz, then I 
wouldn't have to wait.

It's only bugging me because it's too fast to go interrupt driven, but I 
don't have enough waste cycles to do anything useful.

I'm also taking data from the host at 800kbits/sec, and doing a fun pixel 
count.
I have to count, in a 104 byte string, how many total black bits, and how 
many under each of 0x88, 0x44, 0x22, and 0x11 masks.
Then I do compensated burn time calculations for either four burns masked 
like that, or two masked as 0xAA,0x55
I also have to load the head either two or four times per burn line.
Tonights test gives 33 lines/sec print speed, roughly 1.5x the 
competition's top of the line machine..

I had a different problem though. I asked them to put resistors in series 
with the SPI clock and data leads, to keep EMI down..
They put in values 50 TIMES larger than I asked for..
Input is a sharp square wave, output looks like wiggling a rope..

>When I switch over to my manual bit banging, I needed to insert a
>small delay to slow down the clock toggling for reliable
>communication.  Right at the border for when it works and when it
>doesn't work, the clock is at around 36 KHz - significantly slower
>than the slowest I can make the hardware SPI clock run.

check that the polarity is right, it might be loosing it in setup time.

>So it looks like the problem is that the slave device is too slow to
>keep up, and that I cannot prescale the hardware SPI clock down
>enough.  Bummer.

My first version of the printer was software driven, no spi.
I was so pushed for speed, that I didn't loop the byte, or the line routines.
104 straightlined calls to a straightlined byte output engine.
Ouch.


>The device I'm hooking up, BTW, is a Sony PS/2 gamepad.
>
>It's still possible I'm doing something wrong.  I'd really like to use
>the hardware SPI if possible, it seems much more elegant than bit
>banging it.  But whatever works, I guess.

Well, you can always underclock, or use the FDIV register.