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11.2.2
DTMF Load Register (DTLR)
Bit
7
6
5
4
3
2
1
0
—
—
—
DTL4
DTL3
DTL2
DTL1
DTL0
Initial value
1
1
1
0
0
0
0
0
Read/Write
—
—
—
R/W
R/W
R/W
R/W
R/W
DTLR is an 8-bit read/write register that specifies the ratio by which the clock frequency at the
OSC pins is divided for input to the DTMF generator.
Upon reset, DTLR is initialized to H'E0.
Bits 7 to 5—Reserved Bits: Bits 7 to 5 are reserved: they are always read as 1, and cannot be
modified.
Bits 4 to 0—OSC Clock Division Ratio 4 to 0 (DTL4 to DTL0): Bits 4 to 0 specify a division
ratio of the OSC clock frequency which will generate a 400-kHz clock for input to the DTMF
generator. The ratio is set as a counter value from 3 to 25, corresponding to OSC clock frequencies
of 1.2 to 10 MHz (in 400-kHz steps).
Description
Bit 4:
DTL4
Bit 3:
DTL3
Bit 2:
DTL2
Bit 1:
DTL1
Bit 0:
DTL0
Division Ratio
OSC Clock
Frequency
0
0
0
0
0
Illegal setting
(initial value)
1
Illegal setting
1
0
Illegal setting
1
3
1.2 MHz
1
0
0
4
1.6 MHz
:
:
:
:
:
:
:
1
1
0
0
1
25
10 MHz
1
*
Illegal setting
1
*
*
Illegal setting
Note:
*
Don’t care
These bits must be set to the correct value. Normal DTMF signal output frequencies will not be
obtained if these bits are set to a value not matching the clock input at the OSC pins. Operation is
not guaranteed if these bits are set to a value other than 3 to 25.