Figure 5. PICVIDEO on breadboard.
ABOUT THE AUTHOR
Robert Lang is a professional electrical engineer interested in
embedded microprocessors, MIDI, and music. He has written several
articles for electronic hobbyist, computer, and synthesizer magazines.
He can be reached at firstname.lastname@example.org
Two things were done to try to minimize the hula
action. First, the microprocessor was pushed to its maximum clock speed and beyond. I operated the microprocessor at 24 MHz. Second, I doubled the width of the
screen characters to minimize the hula. The dot stretching
is accomplished by inserting no-operation instruction
(NOP) after each dot display is started on the screen. I
also tried several variations on a phase locked loop, but
these were unsuccessful. One of the pitfalls of using a
non-standard clock speed is that the value of SPBRG has
to be estimated from the MicroChip equation:
Baudrate = Clockspeed / ( 16 (SPBRG+1))
I came up with the value of SPBRG to equal 77, and BRGH
to equal one. This gives an approximate 19200 baud rate
on the serial line using a 24 MHz microprocessor clock.
Now, with the hardware built, we are ready to program
the microprocessor and test the unit, which we will do
next month. While the TV typewriter is functional, more
useful applications include superimposing information on
the video screen, such as the time or temperature. In “PIC
Video, Part 2,” we will discuss some possible applications
for PICVIDEO and explore the wonderful world of subliminal messaging. NV
1/4-watt, 680-Kilohm resistor
R2, R3, R5, R6, R7
1/4-watt, 580-Ω resistor
LM1881N video sync separator
Maxim233 dual RS-232 transmitter/receiver
24 MHz crystal
SPST switch (NORMAL)
Momentary contact switch (RESET)
RF modulator (if your TV does not have a
video input and you have no video recorder)
Table 2. PICVIDEO parts list.
Figure 6. Problems with the 20 MHz crystal.
Figure 7. Working PICVIDEO.
NUTS & VOLTS