Bare metal

I have written about how its all connected in Design.

All data passed through shared memory will be read as an object of type shared_data_t. The object contains checksum and and sequence counter. This ensures we aren't reading stale data and no corruption has occurred.

# shared_data.h
typedef struct {
    uint32_t magic;          // always SHARED_MAGIC, written once at init
    uint32_t seq;            // incremented every write by CPU0
    uint32_t servo_target[6];
    uint32_t checksum;       // seq ^ servo_target[0] ^ 0xCAFEBABE
} shared_data_t;

Ensure TLB rule is set up such that program doesnt cache any of its data reads from L1 and L2 cache. Typically you will need to do Xil_DCacheInvalidate() to invalidate L1 cache but TLB rule handles all of that and more.

The servo MG996 accepts pwm signals from 0.5ms to 2.5ms for its angle. It looks something like this: 0.5ms = 0 degrees 1.25ms = 90 degrees 2.5ms = 180 degrees However, my program doesnt take time as input; instead, it uses ticks. My Processing System runs at 50MHz clock so there are 25,000 ticks for every 0.5ms interval. With this information, I can derive this equation:

(uint32_t)(25000.0f + (angle / 180.0f) * 100000.0f)

Using xil_io, I can write directly AXI. Base address for the servo controller for me was 0x43C00000. Each input is offset by 4. With all this, we can read from shared memory, set Translation Lookaside Buffer, and control the servo.