COM 0 and COM1 are pr ogrammable for baud rates
between 110 and 38400. (W hen you refer to the baud
rate tables, the cpu clock is 19.6608 Mhz.) Other
programming capability include data length at 7 or 8
bits, 1 or 2 stop bits, and none, even or odd parity.
Initialization
The CM ON BIOS initializes COM0 on pow er up or
reset and auto-run is not selected. It is a good idea to
initialize COM 0 in your prog ram if you intend to use
this port during run time. T he problem is when you
auto-run your program after a power up, COM 0 won' t
get initialized. Initialized settings are not changed on a
push button r eset.
COM0 is normally used as a debugging port. However,
during run time it may be used as any other serial port
provided you do not use the GD B debugger. You must
use HINT or the device you intend to connect to COM 0.
You may use the GD B debugger if you don' t care about
any output from COM 0.
All of the demonstration programs use either C OM0 or
COM 1. Initialization routines for COM1 ar e in the
400IO sub-directory and file. The 400IO.C file also has
general purpose get and put character and string
routines.
RTS and CTS Lines
CTS and RTS lines are u sed for ha rdwa re flow control.
The SH-1 processor does not have these lines as part of
its UART. They are brought out digital ports on the
CPU where its function may be emulated.
Names for C TS and RTS lines is a source of confusion.
Normally, CTS is an output from a PC. On the RPC-
400, CTS is an input on COM0 and an output on CO M1-
COM 3. Conversely, CTS is a PC input while it is an
output on COM 0. T he discussion that follows will use
the PC convention. Keep in m ind that the names and
functions are reversed on CO M0 only.
RTS is used by a receiver to indicate if "OK to send". A
high level at the connector indicates yes, ' send data'
while a low level indicates ' hold off' . T his line goes to
a sender CTS pin to signal conditions stated above.
The CTS input line on COM1 and RTS input on COM0
have a 4. 7K ohm p ull up to + 5V at the co nnector. T his
is to enable communication if this line was missing on
the external device.
CTS works nicely as a hardware hold-off. It can
generate an interrupt when it goes high and there are
charac ters to tra nsmit. Howe ver, when C TS chec king is
operated in polled mode, the line must be checked every
time a character is transmitted. Should the receiver
signal "h old off" , the progr am m ust wait further until
CTS goes back high.
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