The myRIO C library

The T1 target computer, the NI myRIO, has an associated C library that is included in the D1 VM distribution workspace under the C Support for myRIO directory. This contains several helpful functions and data types in files under source for interacting with myRIO I/O. For speed, nearly all of the I/O is handled by the FPGA. The FPGA comes preconfigured with a “personality” (i.e., a bitstream configuration file) that defines how C programs running on the microprocessor (i.e., the programs we write) can access the I/O. Interfacing with the I/O through the FPGA comprises the majority of the myRIO’s C library.

Of special importance is the header file MyRio.h, which we will include in each program written for the myRIO: #include "MyRio.h". This header file includes MyRio1900.h, which in turn includes NiFpga.h, which declares the primary I/O data types and functions. Whenever we want to open a myRIO FPGA session, we call

NiFpga_Status status;         // declare status type
status = MyRio_Open();        // open myRIO FPGA session

The NiFpga_Status type is defined in the NiFpga.h header file with

typedef int32_t NiFpga_Status;

The C facility typedef creates a new data type (e.g., int, double) in terms of some other data type. In this sense, it is more of an alias than a proper new data type. In the typedef code given previously, NiFpga_Status is defined to be synonymous with the built-in C type int32_t, a fixed-width integer of width 32 bits.¹ A value of 0 indicates proper status, negative integers correspond to error codes, and positive integers are warnings.

The MyRio_Open() function’s prototype is declared in MyRio.h as follows:

NiFpga_Status MyRio_Open();

Its function is to open a communication session between the microprocessor and the FPGA. This must be called before any other FPGA function.

As we have already mentioned, an NiFpga_Status of 0 indicates the successful opening of a session. The constant NiFpga_Status_Success is the name for this. There are several other NiFpga_Statuses defined in NiFpga.h, like the error status NiFpga_Status_MemoryFull, which has integer value -52000.

It is important to test the status after calling MyRio_Open(). In MyRio.h, the function MyRio_IsNotSuccess() is defined. It accepts an argument of type NiFpga_Status and returns another custom type, NiFpga_Bool, defined as

typedef uint8_t NiFpga_Bool;

This is the C unsigned eight-bit integer type and can take on the value NiFpga_False (0) or NiFpga_True (1). Assigning names to logical true and false is common practice in C, which contents itself with 0 and 1 to serve this purpose. Testing for the successful opening of an FPGA session, then, is a matter of

status = MyRio_Open();            // open myRIO FPGA session
if (MyRio_IsNotSuccess(status)) { // test if sess. opened successfully
  return status;                  // if not, return error/warning code
}

Placing this in a main() function attempts to open an FPGA session, but if it is not successful, the execution will terminate and return an error code. Given that I/O is necessary for virtually all interesting programming of the myRIO, these statements should appear in all main() programs that we write.

An open FPGA session should be closed before the end of main(). The session can be closed with

status = MyRio_Close();        // close myRIO FPGA session

Again, a nonzero returned status is an error code or warning.

Of course, after we open a session and before we close it, we will want to actually interact with the myRIO I/O through the FPGA. In the lab exercises that follow, we will do just that!