User guide

6: ICP Software
DC 900-1338I 109
6.3 Interrupt Service
At the interrupt level, the SPS provides specific examples of SCC and IUSC program-
ming for asynchronous (ASYNC), byte synchronous (BSC), and bit synchronous
(HDLC/SDLC) modes of operation. At the same time, examples are provided for:
• operation with and without the use of DMA
• C and assembly language programming
• CRC calculation in hardware (by the SCC or IUSC) or in software
Ta bl e 6 –1 summarizes these features for each mode of operation.
6.3.1 ISR Operation in HDLC/SDLC Mode
In HDLC/SDLC mode, DMA is used for both transmit and receive. The SCC or IUSC
automatically provides the opening and closing flags on transmit. The DMA transfer
count is set to the number of bytes in the frame, not including CRC and flags. The SCC
or IUSC is set to calculate the CRC during transmission of the frame and to send the
CRC when it detects a transmit underrun. When the DMA reaches terminal count (and
Table 6–1: Summary of Communication Modes
Asynchronous BSC HDLC/SDLC
SCC or IUSC mode Asynchronous Byte synchronous Bit synchronous
Data transfer method Character interrupts Character
interrupts/DMA
DMA
Start block detection
(receive)
ISR search for start
character
SCC or IUSC detects
SYNC character
SCC or IUSC detects
opening flag
End block detection
(receive)
ISR search for end
character
Byte count in header SCC or IUSC detects
closing flag
CRC calculation Software Software SCC or IUSC
ISR programming
language
C Assembly/C C