Pulse-Width Tone
7-55
MPU Public Peripherals
The clock behind the multiplexer is divided by 154 to get the required frequen-
cies on the TONE output. The 12 frequencies in an octave can be programmed
with bits 5 to 2 of the frequency control register (FRC), and the octave can be
programmed with bits 1 to 0 of the FRC. Forty-eight different frequencies can
be programmed subdivided into four octaves with twelve frequencies. The four
frequency dividers with the complex coefficients
101
/
107
,
49
/
55
,
50
/
63,
and
80
/
127
work with the fade out principle. To get the divider
101
/
107
from a periodic pulse,
6 pulses every 107 pulses are fade out. Over a long time the resulting frequen-
cy is
101
/
107
. The resulting signal has two different periods, which differ by one
period of the original signal. Because of this difference, the resulting signal has
jitter. To minimize this jitter, the divider works with high frequencies resulting
in short periods producing low jitter (see Table 7–48).
Table 7–48. Buzzer Frequencies
FRC Bits 5 - 2 1 - 0
Buzzer Frequency
FRC Bits 5 - 2 1 - 0
Buzzer Frequency
0000 00
4868 Hz
0000 10
1217 Hz
0001 00
4595 Hz
0001 10
1149 Hz
0010 00
4337 Hz
0010 10
1084 Hz
0011 00
4093 Hz
0011 10
1023 Hz
0100 00
3864 Hz
0100 10
966 Hz
0101 00
3647 Hz
0101 10
912 Hz
0110 00
3442 Hz
0110 10
860 Hz
0111 00
3249 Hz
0111 10
812 Hz
1000 00
3066 Hz
1000 10
767 Hz
1001 00
2894 Hz
1001 10
723 Hz
1010 00
2732 Hz
1010 10
683 Hz
1011 00
2579 Hz
1011 10
644 Hz
0000 01
2434 Hz
0000 11
608 Hz
0001 01
2297 Hz
0001 11
574 Hz
0010 01
2168 Hz
0010 11
541 Hz
0011 01
2046 Hz
0011 11
511 Hz
0100 01
1932 Hz
0100 11
483 Hz
0101 01
1824 Hz
0101 11
456 Hz