uses the PRU to generate a waveform at up to 2MHz.

first 8 bits of the PRU gpio bus:

• p0 p9_31
• p1 p9_29
• p2 p9_30
• p3 p9_28
• p4 p9_42
• p5 p9_27
• p6 p9_41
• p7 p9_25

connect to the r2r ladder:

Image thanks to www.ikalogic.com

• I used 20k and 10k resistors
• I didn't use the opamp buffer.
• Recommended part

Edit /boot/uEnv, change line 47 to disable HDMI:

##Disable HDMI (v3.8.x)
cape_disable=capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN

Reboot, and check slots:

cat /sys/devices/bone_capemgr.9/slots

Showing only last 2 lines:

5: ff:P-O-- Bone-Black-HDMI,00A0,Texas Instrument,BB-BONELT-HDMI
6: ff:P-O-- Bone-Black-HDMIN,00A0,Texas Instrument,BB-BONELT-HDMIN

• ff:P-O-- = Disabled
• ff:P-O-L = Enabled

cd src
./make install
echo r2r-dac > /sys/devices/bone_capemgr.9/slots 

There should be no error after the last command.

cat /sys/devices/bone_capemgr.9/slots

Showing last line:

7: ff:P-O-L Override Board Name,00A0,Override Manuf,EBB-PRU-Example

cd src
make

Python code generates a 4000 sample saw tooth

./generate_saw.py

./load_data

Now there should be a 3.3v peak to peak ~640Hz sawtooth waveform on the end of the r2r DAC.

more detailed notes here

Many thanks to excellent resource by Derek Molloy.