Sub-Audio Signaling Processor
Page 14 of 24
MX805A
5.2.4.1.1 The Integer (N) –Byte 1
The binary number representing twice the number of complete input sub-audio cycle periods counted during
the measurement period of 122.64ms (4.0MHz crystal)
5.2.4.1.2 The Remainder (R) –Byte 0
A binary number representing the remainder part, R, of 2x the 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 (4.0MHz crystal)
(Reply Data)
(MSB) - Transmitted First
(Reply Data)
(LSB) - Transmitted Last
Byte 1
Byte 0
15
9
7
1
14
13
12
11
10
8
6
5
4
3
2
0
Integer (N)
Remainder (R)
"0"
"0"
"0"
"0"
CTCSS RX Frequency Register
Figure 6: Format of the CTCSS RX Frequency Register
5.2.4.2 CTCSS RX Frequency Register
The format of the CTCSS RX Frequency Register is shown in Figure 6.
Bits 8 (LSB) to 13 (MSB) are used to represent the Integer (N). From Byte 1, valid values of
N = 16 ≤ N ≤ 61 i.e. values of N less than 16 and greater than 61 are not within the specified frequency band.
Bits 0 (LSB) to 5 (MSB) 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 (01H) and may adopt any random configuration at
Power-UP.
5.2.4.3 CTCSS RX Frequency Measurement Formulas
To assist in the production of “lookup” tables and limit-values in the microcontroller, and to provide guidance
upon the determination of N and R from a measured CTCSS frequency, the following formulas show the
derivation of the CTCSS RX Frequency (f
) from the measured data bytes (N and R).
CTCSS IN
5.2.4.3.1
f
CTCSS IN
In the measurement period of 122.64ms there are N cycles at 2 x f
and R clcok cycles at 4166.6Hz,
CTCSS IN
for any input frequency.
N× fXTAL
1920×(511-R)
fCTCSSIN
=
é
CTCSSIN ù
1920 x 511xf
fXTAL
N = INT
ê
ú
û
ë
é
ù
ú
ú
û
æ
ö
÷
÷
ø
N x fXTAL
1920 x fCTCSSIN
ç
ç
è
R =INT 511-
+ .5
ê
ê
ë
Calculate N first
Example: (f
= 4.00Mhz): f
= 100Hz N = 24 R = 11; f
= 250Hz N = 61 R = 3
CTCSS IN
XTAL
CTCSS IN
5.2.4.4 NoTone Timing
The input sub-audio signal is monitored by the Frequency Assessment Circuitry. Before any NoTone action is
enabled, the MX805A 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 NoTone timer will start to charge via the
timing components. When the timing period has expired (at V /2), an Interrupt and a Status bit (NoTone
DD
Timer Expired) are generated. This is a one-shot function which is rest by a “Tone Measurement Complete”
interrupt.
ã2001 MX-COM, Inc.
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480116.006
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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