You have a tiger by the tail there Simone.
Asynchronous serial communications uses fixed bit timing with word preamble
and postamble (typically, start and stop bitd) and does not send a clock
signal along with the data.
Synchronous serial communications (HDLC, etc) uses a clock signal the is
either generated or received by the USART.
The difference between the two is significant. What kind of media is being
used? RS232? If so, then you want asynchronous.
DMA:
Direct Memory Access. You can transmit data out your USART by writing values
to a register, which is sometimes used, but is inefficient if your CPU has
other things to do, especially at high serial bit rates. The alternative is
to program a device callled a DMA controller to do the writting (and
receiving) for you. Its complex though, requiring proper initialization of
the USART, DMA controller, and memory spaces. The CPU writes a data pattern
to a memory block and then tell the DMA controller to begin transmitting. It
feeds the memory block to the USART as the required pace.
IRQ
Interrupt Request. Your USART can probably be programmed to issue a hardware
interrupt when either the transmitter or receiver needs attention or a comm
error has been detected. The interrupt causes an Interrupt Service Routine
to be executed, designed to handle the USART. Or, the DMA Controller can
also be programmed to issue an interrupt when a block of data has been
received or transmitted. Again, an ISR will sevice the DMAC and keep it feed
with data, etc. In all cases the hardware interrupt subsystem must be setup
to recognize and handle the interrupts issued by these devices.
Tired yet?
The USART probably also has optional FIFOs that allow for the preloading of
transmity data and storae of receive data. Thats another level of
complexity.
Bursting.
Bursting is a term that simply means to transfer two or more words of data
back-to-back, ie: no delay between transfers.
Hope this helps.
Dana Frank Raymond
