GPIO LED blinker using SYSFS on the Rock64

[MarkHaysHarris777]

This chart shows the pins ( see chart ) that have been tested, and whether they work;

Green PI-2 Bus 
pin( 7)    GPIO1_D4    NO       ( this pin will crash the Rock64, you've been warned )
pin(12)    GPIO2_A3    NO       ( this pin should work, but does not ) (note: it does work as input!)
pin(13)    GPIO0_A0    NO       ( this pin is for toggling power to the usb3.0 port, per p19 of Rock64! )
pin(31)    GPIO0_A2    NO       ( pin(31) should not be used, but for completeness is for the usb2.0 ports )
pin(15)    GPIO3_A4    YES     gpio100
pin(16)    GPIO3_A5    YES     gpio101
pin(18)    GPIO3_A6    YES     gpio102
pin(22)    GPIO3_A7    YES     gpio103

Green PI P5+  Bus 
pin( 3)    GPIO2_C1    YES     gpio81
pin( 4)    GPIO2_C2    YES     gpio82
pin( 5)    GPIO2_C7    YES     gpio87
pin( 6)    GPIO2_C3    YES     gpio83
pin( 9)    GPIO2_C0    YES     gpio80
pin(10)    GPIO2_B7    YES    gpio79
pin(11)    GPIO2_C5    YES    gpio85
pin(12)    GPIO2_C4    YES    gpio84
pin(14)    GPIO2_C6    YES    gpio86

Yellow PI P5+  Bus 
pin(21)    GPIO2_D1    YES    gpio89
pin(22)    GPIO2_D0    YES    gpio88

Blue PI-2 Bus      ( use the blue pins ONLY if you are booting from eMMC and NOT using micro SD card )
pin(33)    GPIO1_A0    YES    gpio32
pin(35)    GPIO1_A1    YES    gpio33
pin(37)    GPIO1_A2    YES    gpio34
pin(40)    GPIO1_A3    YES    gpio35
pin(38)    GPIO1_A4    YES    gpio36
pin(36)    GPIO1_A5    YES    gpio37
pin(32)    GPIO1_A6    YES    gpio38

The Rock64 seems to have a minor design flaw for the maker community, in that the available gpio hardware pins are already used internally by the system and only a very few of them are available for programming. This may require expanding the gpio bus with external shift registers|latches or SPI | I2C expanders. 
This is somewhat mitigated by the fact that some services like I2S or ethernet 10|100 may not be in use and would theoretically be available.

WARNING:  DO NOT experiment with any of the other pins at this time;  they will crash the Rock64 or damage the existing image on the SD card or damage the image on the eMMC module!