Most engineers will recognise the problem: Your circuit needs a
stable 1 or 2 MHz clock generator (in the author’s case it was for a
Pong game using an old AY3-8500). A suitable crystal is not to hand so
you cobble together an RC oscillator (there are plenty of circuits for
such a design). Now it turns out that you don’t have exactly the right
capacitor so a preset pot is add e d to allow some adjustment . Before
you know it the clock circuit is taking up more space on the board than
you had hoped.
Providing the application does not demand a
precise clock source a tiny 8-pin microcontroller may offer a better
solution to the problem. It needs no additional external components and
an old ATtiny15 can be found quite cheaply. Another advantage of the
solution is that clock frequency adjustment does not involve changing
external components and is not subject to component tolerances.
The
microcontroller’s internal RC oscillator is already accurately
calibrated to 1.6 MHz. With its inbuilt PLL, internal Timer 1 can
achieve up to 25.6 MHz [2]. By configuring internal dividers the timer
can output a frequency in range of roughly 50 kHz up to 12 MHz from an
output pin. The difference between calculated and the actual output
frequency increases at higher frequencies. A meaningful upper limit of
about 2 MHz is a practical value and even at this frequency the
deviation from the calculated value is about 15 %.
MHz Oscillator using an ATtiny15 Schematic
The
circuit diagram could hardly be simpler, aside from the power supply
connections the output signal on pin 6 (PB1) is the only other
connection necessary.The example program, written in Assembler is just
15 lines long! With a program this short comments are almost super
fluous but are included for clarity. The code can be downloaded from the
Elektor website [1].
The program only needs to initialise the
timer which then runs independently of processor control to output the
clock sign al . The processor can then be put into sleep mode to memory
used up the remaining 99 % is free for use for other tasks if required.
The
OSCCAL register contains a calibration byte which allows some
adjustment of the CPU clock. This gives a certain degree of fine tuning
of the output frequency. A recommendation in the Atmel data sheet
indicates that the CPU clock frequency should not be greater than 1.75
MHz otherwise timer operation cannot be guaranteed.
The more
recent ATtiny45 can be substituted for the ATtiny15. In this case the CK
SEL fuses should be set to put the chip’s Timer 1 into ATtiny15-
compatible mode [3]. After adjustment to the program it will now be
possible to obtain a higher (or more exact) frequency from the timer,
the ATtiny45’s PLL can operate up to 64 MHz. Link
