AN2120: Connecting an M68HC08 Family Microcontroller to an ...

Application Note
Listing 2 The listing shows the modem service routine that must be called from
the ISR of the SCI driver.
The method just described allows great flexibility while handling the
FIFO. For instance, to retrieve the number of characters stored in the
FIFO, the software only needs to subtract dataSlot from emptySlot.
Another example is the operation to flush the contents of the FIFO will
simply require the statement dataSlot = emptySlot.
The code to dequeue (pull out of queue) a character from the FIFO is
shown in Figure 28.
BYTE ModemGetch (void) {
BYTE c = 0;
if (mDataSlot != mEmptySlot) {
c = ModemBuffer [mDataSlot];
mDataSlot++;
if (mDataSlot > MODEM_BUFFER_SIZE) mDataSlot = 0;
return(c);
}
else {
return (BYTE)0x00;
}
}
Figure 28. Code to Dequeue a Character from the Modem FIFO
Listing 3 Two important functions are also defined inside the modem module: the
Transmit() and Waitfor() functions. The first transmits data to the modem
while the second waits for any particular character or a string of
characters before it times out. When used together, both functions
provide support for complex scripts required for SLIP sessions.
Obviously, those scripts will be built in the ROM code, making it difficult
to maintain in some applications.
AN2120
38 MOTOROLA