Nuts and Volts - May 2015

The following information explains the chosen value for each bit: bit0: Receive background serial data to the scratchpad [1 = use scratchpad]. We must use 0 because M2 processors don’t contain a scratchpad. bit1: Invert serial output data [1 = negative]. We’ll use 0 [true] for output because M2 processors must use “true” for input. (See bit2 next.) bit2: Invert serial input data [1 = negative]. We must use 0 [true] for input because M2 processors are not able to receive inverted serial data. bit3: Disable hserout [1 = hserout pin is normal I/O]. We’ll use 1 [disable] because we’re not going to use the hserout pin for that purpose. bit4: Disable hserin [1 = hserin pin is normal I/O]. We’ll use 0 [don’t disable] because we’re going to use the hserin pin for that purpose.

various special functions of the hardware serial port. In the next experiment, we’re going to use %01000 for the mode parameter.

As we discussed previously, M2 processors do not have a scratchpad storage area; instead, they use an internal two-byte first-in first-out (FIFO) storage buffer. A maximum of two bytes can be received in the background at any time — not just when the hserin command is executed. If more than two bytes are received before they are processed, the extra ones are lost.

To summarize all the above information, the hsersetup statement that we will use in the next experiment is hsersetup B9600_ 8, %01000.

Understanding the M2- Class Hserin Command

Now that we’ve specified our hsersetup statement, we can move on to discussing the M2-class hserin command. The syntax is mercifully simple: It’s just hserin myVar, where myVar is the variable that receives the incoming serial data byte. However, the question of how the incoming background-received data byte actually gets transferred to the variable is a little more complicated.

Whenever an hserin command is executed, the first byte in the buffer is transferred to the myVar variable. If there is a second byte in the buffer, it’s moved up to the first position. Finally, if the buffer is empty, the value of myVar is not changed.