Controlling Protocol ......
“Read CTCSS Rx Frequency Register”
......
The Integer (N)
– Byte 1
A binary number representing ‘twice the number of
complete input sub-audio cycle periods’ counted during the
measurement period of 122.64ms
The Remainder (R)
– Byte 0
A binary number representing the remainder part, R,
of 2 x Sub-Audio Input Frequency. ‘R = number of specified
measuring-clock cycles’ required to complete the specified
measurement period (See N). The clock-cycle frequency is
4166.6Hz
(REPLY DATA)
(MSB) – TRANSMITTED FIRST
Byte 1
12
11
10
9
8
7
6
5
Byte 0
4
3
(REPLY DATA)
(LSB) – TRANSMITTED LAST
15
14
13
2
1
0
“0” “0”
Integer (N)
“0” “0”
Remainder (R)
Fig. 6 Format of the CTCSS Rx Frequency Register
CTCSS Rx Frequency Register
Figure 6 (above) shows the format of the CTCSS Rx
Frequency Register.
Bits 8 (
LSB
) to 13 (
MSB
) are used to represent the Integer
(N). From Byte 1, valid values of N = 16
≤
N
≤
61.
ie. values of N less than 16 and greater than 61 are not
within the specified frequency band.
Bits 0 (
LSB
) to 5 (
MSB
) (Byte 0) are used to represent the
Remainder (R). From Byte 0, valid values of R =
≤
31.
This register is not affected by the General Reset
command (01
H
) and may adopt any random configuration at
Power-Up.
CTCSS Rx Frequency Measurement Formulæ
To assist in the production of ‘look-up’ tables and limit-values in the
µController
and provide guidance upon the determination
of N and R from a measured CTCSS frequency, the following formulæ show the derivation of the CTCSS Rx Frequency
(f
CTCSS IN
) from the measured data bytes (N and R).
f
CTCSS IN
In the measurement period of 122.64ms there are N
cycles at 2 x f
CTCSS IN
and R clock-cycles at 4166.6Hz, for any
input frequency.
So
f
CTCSS IN
=
N x f
XTAL
1920 x (511 -R)
N =
INT (1920 x 511 x f
CTCSS IN
)
f
XTAL
Calculate N first
Examples
(f
XTAL
= 4.00MHz)
:
f
CTCSS IN
= 100Hz N = 24 R = 11; f
CTCSS IN
= 250Hz N = 61 R = 3
[2]
Hz
[1]
R
=
INT
511 –
N x f
XTAL
1920 x f
CT CSS IN
+ 0.5
[3]
N
OTONE
Timing
The input sub-audio signal is monitored by the Frequency
Assessment circuitry. Before any N
OTONE
action is enabled,
the FX805 must have achieved at least one successful “Tone
Measurement Complete” action.
If there is no signal or the signal is of a consistently poor
quality, the N
OTONE
Timer will start to charge via the timing
components. When the timing period has expired (at V
DD
/2),
an Interrupt and a Status bit (N
OTONE
Timer Expired) are
generated. This is a one-shot function and is reset by a “Tone
Measurement Complete” interrupt.
9
CMLMICRO [ CML MICROCIRCUITS ]
