Not hard to conceptualize really. Say the pulse that writes data into SPDR also clocks a FF that releases a counter and gates a clock through to a shift reg. The timer allows 8 clock pulses to the SR then resets the FF which stops the SR and counter.

Result, data gone out the MOSI pin.

_BV = Bit Value, a macro I think that will eveluate to 1000000 or bit 7 which is the location of the SPIF flag.

So the code is ANDing the SPSR reg with 1000000, if the result is 0 then the flag is not set and therefore the byte has not been fully transmitted.

Code:

 SPDR = _data;

load your outgoing data into the SPI shift reg's data register

 

Code:

while (!(SPSR & _BV(SPIF))) ;

wait for the data to be transmitted on MOSI (and also received on MISO)

Code:

 return SPDR;

return the received data from the data register

It physically won't fit.

Can you use an SMD resistor? That can go just about anywhere.

AFAIK it's up to the program to toggle SS, there's no other practical way to do it because the SPI hardware can't know how many bytes you want to send.

If the SPI library toggles for each byte then you can't use it, or you can use it but implement you own SS and leave the normal one (pin 10?) unconnected.

But it seems like the dac only needs two pins to communicate via SPI... because it doesn't need to send data back to the Arduino.  So pin 12 on the Arduino doesn't need to be connected to anything.

True, you don't have to use MISO.

I can't tell if it's needed for the smaller single dac chip I'm using.

The single version still has an LDAC pin, so yes it would be needed.

Unfortunately LDAC is an active LOW latch enable so you can't just hook it up to SS on the SPI interface as SS will go low for the entire frame. You can with the SR because it's a rising-edge clock, but not the DAC.

You need to organise a low pulse (min 100nS) on the rising edge of SS and feed that to LDAC.

1) So do that
2) The SR can shift at about 25MHz, I'm pretty sure it will keep up  
3) Only you can decide this because you know what else the code has to do, but if you're using the SPI hardware there's almost 0 overhead in shifting bits out at 4MHz so it shouldn't affect your code much.

Anyway it's worth a try.

Rob

EDIT: Looks like the SR can shift at about 12.5MHz.

What about daisy-chaining the two chips.

ARD -> serin SR serout -> sdi DAC

clock and latch are in parallel and a single data out pin so only 3 pins needed.

Send 16 bits every time. When writing to the SR send the existing value to the DAC, and vice versa.

Pulling SCLR low will simply cause 0s to be latched to the outputs when RCK goes high.

Unfortunately I don't think it's possible to do this without a small amount of logic, say to gate the SRs clock signal.

If you put it in the buffer surely you can get it out. So is the question really how to upload the data to a PC for graphing? Or are you using a GLCD? What frequency does this need to occur at? Against what time base, time of day or just relative to the last reading? etc etc.

Solder normal headers back-to-back.

If necessary split the ribbon cable and use seperate IDC sockets to handle the odd header spacing.

The 7219 has an SPI-style interface that uses four pins so you can use both.

Maybe many examples are done this way to ease any beginner confustion, HIGH = ON possibly makes more sense for someone just starting out.

I made a start on such a list a couple of weeks ago, see this thread

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1285946134/3#3

or for direct links to the table

http://www.robgray.com/temp/Arduino-shield-pins-used.xls

If you don't trust an XLS file here's a screen shot

http://www.robgray.com/temp/Arduino-shield-pins-used.png

The table details many shiilds and the pins they use.

I spent hours looking at schematics (where available) and also used info from Mowcius' site and the Arduino site, but don't treat the list as gospel, many schematics are confusing plus I got a bit cross-eyed after a while. Also some schematics were in a version of Eagle I coudn't read.

Hopefully one day we can build on this to provide a comprehensive resource.

Rob

200 bar! Are you sending an Arduino to the bottom of the Puerto Rico Trench?

I would think that at least some components would implode, for example electro caps.

Why does everything have to be under pressure, are you not allowed to pierce the vessel or something?

There's a thread here about the different pins used by shields

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1284962949/5#5

with a table (as yet unverified) showing the pins used by a lot of shields.