Holtek HT-IDE, User Manual

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Using an I/O Port as a Serial Application
This section shows code to simulate serial port operation. This can be used
as a basis for the development of simple serial port applications such as
8-bit communication, non-parity, single stop bit applications. Here the
example is given as a project with the user left to decide which pins are send
and receive and the baudrate decided by the system frequency. Below are
the steps taken and other points to be noted.
•
Create a new file called DFSERIPT.INC in the directory \HT-IDE\IN-
CLUDE\Here must be included ...
–
Baudrateconst XXX. This value XXX can be taken from a table or
calculated (ignore the decimal point)
–
TXPIN ...... the defined transmit pin
–
RXPIN ...... the defined receive pin
•
The serial port program uses 4 RAM locations, 2 I/O pins and 49 program
memory locations. The TXPIN must be defined as an output and RXPIN
defined as an input.
•
Before Call or Receive, the RXPINs stop bit condition must be checked
Calling these two subroutines will change the condition of the carry flag.
•
When using Routines, the program must first declare them as follows:
EXTERN TRANSMIT: NEAR
EXTERN RECEIVE: NEAR
•
Below the main program serial.asm must be added.
The parameter baudrateconst is calculated from the baudrate as follows.
Baudrateconst = (Fsys/(baudrate*12))-3 (ignore the decimal point)
Note
The best value for baudrateconst is within a range of 7 to 256, the bigger the
value the smaller the error. The following table gives some direct values.
Refer to the table below:
Baudrate/Fsys 4MHz 2MHz 1MHz
9600 31 14 X
7200 43 20 8
4800 66 31 14
3600 89 43 20
2400 135 66 31
2000 163 80 38
1800 182 89 43
1200 X 135 66
Chapter 15 Input/Output Applications
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