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| | | PicPuter 2.0 : Appendix D : Serial Port |
PicPuter contains a serial port that runs at 115200 baud by default, but can run at any common baud rate between 115200 to 1200.
There are several commands to designed to perform IO on the serial port in basic.
The commands that input data from the serial port are INPUT and GETCHAR .
The commands used to output data to the serial port are
PRINT , CPRINT , HPRINT , and FPRINT .
The serial ports output is buffered through a 16 byte fifo. This allows basic statements to be run while the serial port data shifts out. If the fifo is full, the basic program stalls until the fifo empties a bit. At 115.2K baud the fifo does not seem to fill up much, but at lower rates it becomes more important.
While not currently in the code, an assembler vector table will be added to give an assembler routine access to the serial port by V2.0.
 Hooking up the Serial Port. |
PicPuter will operate with or without and handshaking. If you opt not to hook up the CTS/RTS lines you will not be able to cut and paste examples or use the failsafe bootloader . Also PortA.4 will need to be grounded to see any output.
If handshaking is hooked up then you can use a send text file command....
| Using other Operating Systems/ USB |
PicPuter has been tested using a windows PC and a macintosh.
On the PC, I have noticed that my old >1MHz development box had trouble keeping up at 115.2K baud. It would not respond to a ^C while doing a listing, I thought it was a bug but it seems work on my new development box. If PicPuter seems to not respond to ^C or Esc then reduce the default baud rate.
The terminal program that everbody uses since it is installed with windows is Hyper Terminal. This program does not correctly handle software flow control (XON/XOFF). Because of this PicPuter was forced to burn two pins implimenting hardware handshaking (RTS/CTS) which it seems to work correctly. An old outdated software flow control version worked fine on my mac but not on the windows box. I would like to personally thank MicroSoft and HillGrave for forcing this buggy piece of software on consumers and making it the standard. I will write a less buggy one for a $0.01 royality, just __CONTACT__ me.
Since my Machintosh does not have a serial port, I use a __KEYSPAN__ usb serial port. It works great and has drivers written for windows, linux, as well as apple. The interface/ computer are slow so a baud rate of 115.2K cause RTS/CTS handshaking to cycle, which works fine, but you need to hook it up otherwise the data will overrun.
The OS-X compatiable terminal program I use is __ZTERM__.
| Changing the baud rates |
In order to generate a baud rate without any frequency error, It is important to use a crystal that divides by the baud rate perfectly, otherwise you end up running a rate like 115343 that may not work. It may seem like using a 20MHz crystal is a good idea, but a 18.432MHz crystal can generate the proper baud rate.
Note: A crystal frequency drifts over time.
| Temporary Baud Rate Change |
To make a temporary change you need to alter the
SPBRG register. The Serial Port Baud Rate Generator SPBRG is a typical PicPuter register. You can edit it by using a command like 'SPBRG=19'. Once the command is executed the baud rate changes immediately, you will probably need to alter your terminal program settings in order to reestablish communications with PicPuter. If you are using the default crystal value of 18.432 MHz you can just use the chart at the bottom of the page to determine the correct baud rate.
| Persistent Baud Rate Change |
If you are fortunate enough to be a register picputer user, you can use __AUTORUN__ to execute a statement like '1 SPBRG=19' to automatically change the default baud rate at reset. A clever register picputer user could even create a small program that would automatically determine the baud rate by printing 'press return' looking for a 13 10.
Another way to affect a persistent change is to change the autoconfig script. For V1.80 the default baud rate is located at flash location $07F0. You can alter the flash memory value by using a PicPuter command like '*$7F0=$39xx' where xx is the value in the table below. Since it is located in the lower memory, memory protection will need to be lifted with the SETCON15 contraction .
** Doucmentation error 56K baud incorrect, fixed 11/03 **
3 steps to change the default baud rate (if using a 18.432MHz Crystal):
SETCON15 ; turn off protection of lower memory
*$7F0 = $39xx ; Lookup xx in the table above.
CLRCON15 ; turn on protection of lower memory.
Note: A divide by 4 bit is in the TXSTA register to get rates below 4800.
If you mess up upload these hex records in the builtin bootloader
:020000040000FA
:100FD0001700823F3F250126902700282804DF00C4
:100FE00009395F32240478000F1D0F1730156F0484
:100FF0002B001F204004FF00FF201C12022CC00BFE
:02400E00763FFB
:00000001FF
OR
:020000040000FA
:100FD0001700823F3F250126902700282804DF00C4
:100FE00013395F32240478000F1D0F1730156F047A
:100FF0002B001F204004FF00FF201C12022CC00BFE
:02400E00763FFB
:00000001FF
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