Document SPI loopback test

Author: pdp7

doc/spi_loopback_test.md

@@ -0,0 +1,63 @@
+# Adafruit_BBIO SPI test
+
+## Loopback test with SPI0
+* Connect wire from P9_18 [SPI0_D1] and P9_21 [SPI0_D0]
+* Reference: [P9 header pins](http://beagleboard.org/static/images/cape-headers-spi.png)
+
+## System Info
+### uname -r
+`4.4.39-ti-r79`
+### /etc/debian_version 
+`8.6`
+### /etc/dogtag 
+`BeagleBoard.org Debian Image 2016-11-27`
+### /boot/uEnv.txt
+```
+uname_r=4.4.39-ti-r79
+cmdline=coherent_pool=1M quiet cape_universal=enable
+```
+### /sys/devices/platform/bone_capemgr/slots
+```
+ 0: PF----  -1 
+ 1: PF----  -1 
+ 2: PF----  -1 
+ 3: PF----  -1 
+ 4: P-O-L-   0 Override Board Name,00A0,Override Manuf,cape-universaln
+```
+
+### config-pin
+```
+config-pin P9_17 spi
+config-pin P9_18 spi
+config-pin P9_21 spi
+config-pin P9_22 spi
+```
+
+## test-spi.py
+```
+from Adafruit_BBIO.SPI import SPI
+
+# From Adafruit Learning System:
+# https://learn.adafruit.com/setting-up-io-python-library-on-beaglebone-black/spi     
+#spi = SPI(bus, device) #/dev/spidev<bus>.<device>
+#spi = SPI(0,0)	#/dev/spidev1.0
+#spi = SPI(0,1)	#/dev/spidev1.1
+#spi = SPI(1,0)	#/dev/spidev2.0
+#spi = SPI(1,1)	#/dev/spidev2.1
+
+spi = SPI(0,0) 
+print spi.xfer2([32, 11, 110, 22, 220]) 
+spi.close() 
+```
+
+### output
+```
+debian@beaglebone:~/spi$ sudo python test-spi.py 
+[32, 11, 110, 22, 220]
+```
+
+### strace
+```
+debian@beaglebone:~/spi$ sudo strace -e open -f python ./test-spi.py 2>&1 |grep /dev/spidev
+open("/dev/spidev1.0", O_RDWR|O_LARGEFILE) = 3
+```