BA-IS-XX UART-RX

The UART module on the FPGA contains two hardware buffers: for receiving and sending. The size of each buffer is 16 bytes. It is therefore necessary to have a good understanding of the characteristics of the particular UART communication protocol to avoid overflowing these buffers, and to ensure that the previous message is sent before attempting to send the next.

The first thing to consider is the data rate. Obviously, data over the UART is not transmitted instantaneously. Suppose, for example, you want to send an array of 16 characters (each character consists of 10 bits - that’s 8 data bits, start and stop bits) at a rate of 115200 baud. This will take seconds to complete. If the UART Send block runs in 0.001 s increments, the data will not have time to be sent before the next block calculation step because 0.001 < 0.0014. Therefore, the next array of symbols cannot be sent and in such a situation, the execution of the model will stop with an error. It is required to find such a combination of block calculation step, baud rate and array size that it is physically possible to send this data in one block calculation step.

*The second thing to consider is the size of the hardware buffer on receive. If data from outside comes in faster than the BA-IS-XX UART-TX block reads it from the hardware buffer, new data that does not fit in the receive buffer is discarded. Thus, it is necessary to take into account and calculate the expected time to fill the read buffer with an external data stream.

For this purpose, you can be guided by the values at the outputs Length and Overrun block BA-IS-XX UART-TX. In calculations within the model should take into account exactly as many elements at the output RX block, how many were actually received (output Length). The values of all other elements of the RX array are not defined.

If a non-zero value is output at the Overrun block output, it means that the hardware buffer has overflowed and the data in the hardware receive buffer has been lost. In this case, the calculation step of the BA-IS-XX UART-TX block can be reduced in order to read data more frequently and thus avoid overflowing the receive buffer. Too small calculation step for the block BA-IS-XX UART-TX, also should not be set, but should be guided by the specific exchange protocol. For example, if it is known that the external device sends commands of 8 bytes, it is possible to select such a step of calculation of the block that the value of the output Length is approximately equal to 8 from step to step calculation.