ICD2051 Datasheet, PDF (3/8 Pages) Cypress Semiconductor – Dual Programmable Clock Generator

ICD2051:1/95
Revision: April 11, 1995
ICD2051
General Considerations
Programming the ICD2051
The desired output frequency is defined via a serial interface,
with a 22-bin number shifted in. The ICD2051 has two pro-
grammable PLLs (CLKA and CLKB), requiring a 22-bit pro-
gramming word (W) to be loaded into each channel indepen-
dently. This word contains 5 fields:
Table 1. Programming Word Bit Fields
# of
Field
bits
Notes
Index (I)
4 MSB (Most Significant Bits)
P Counter value (P’) 7
Reserved (R)
1 normally set to logic 1
Mux (M)
3
Q Counter value (Q’) 7 LSB (Least Significant Bits)
The frequency of the programmable oscillator f_(VCO) is deter-
mined by these fields as follows:
P’=P−3
Q’=Q−2
f_(VCO)=2 x f_(REF) x P/Q
where f_(REF)=Reference frequency (between 1 MHz − 25 MHz)
The value of f_(VCO) must remain between 40 MHz and 120 MHz.
Therefore, for output frequencies below 40 MHz, f_(VCO) must be
multiplied up into the required range. To accomplish this, a
post-VCO Divisor is selected by setting the values of the Mux field
(M) as follows:
Table 2. Mux Field (M)
M
000
001
010
011
100
101
110
111
Divisor
1
2
4
8
16
32
64
128
The Index field (I) is used to preset the VCO to an appropriate
range. The value for this field should be should be chosen from
Table 3. (Note that this table is referenced to the VCO frequency
f_(VCO), rather than to the desired output frequency.)
Table 3. Index Field (I)
I
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
f_(VCO) (MHz)
40.0 − 42.5
42.5 − 47l.5
47.5 − 53.5
53.5 − 58.5
58.5 − 62.5
62.5 − 68.5
68.5 − 69.0
69.0 − 82.0
82.0 − 87.0
87.0 − 92.0
92.0 − 92.1
92.1 − 105.0
105.0 − 115.0
115.0 − 120.0
115.0 − 120.0
115.0 − 120.0
If the desired VCO frequency lies on a boundary in the table
(if it is exactly the upper limit of one entry and the lower limit of
the next) then either index value may be used (since both limits
are tested), but we recommend using the higher one.
To assist with these calculations, Cypress/IC Designs provides
BitCalc (Part #ICD/BCALC), a Windows™ program which automat-
ically generates the appropriate programming words from the us-
er’s reference input and desired output frequencies. The software
also assembles the program words for control and power-down reg-
isters. Contact your local Cypress representative for more informa-
tion.
Programming Constraints
There are five primary programming constraints the user must
be aware of:
Table 4. Programming Constraints
Parameter
Minimum
f_(REF)
f_(REF)/Q
f_(VCO)
Q
1 MHz
200 kHz
40 MHz
3
P
4
Maximum
25 MHz
1 MHz
120 MHz
129
130
The constraints have to do with trade-offs between optimum
speed and lowest noise, VCO stability and factors affecting the
loop equation. The factors are listed for completeness sake;
however, by using the BitCalc program all of these constraints
become transparent.
ICD2051 Programming Example
The following is an example of the calculations BitCalc per-
forms:
3