18
©2018 Integrated Device Technology, Inc.
August 30, 2018
VersaClock
®
6E Family Register Descriptions and Programming Guide
Crystal Load Capacitor Registers
Registers 0x12 and 0x13 are Crystal X1 and X2 Load capacitor registers respectively that are used to add load capacitance to X1 and X2
respectively. In X1 Switch mode is provided with different mode selection options and in X2 polarity selection of clock can be made whose
values are given in the table.
Figure 2. Crystal Oscillator
Short Example of Programming Crystal
Ci1 and Ci2 are on-chip capacitors that are programmable.
Cs is stray capacitance in the PCB and Ce is external capacitors for frequency fine tuning or for achieving load capacitance values
beyond the range of the on-chip programmability.
All these capacitors combined make the load capacitance for the crystal.
Capacitance on pin X1: Cx1 = Ci1 + Cs1 + Ce1.
Capacitance on pin X2: Cx2 = Ci2 + Cs2 + Ce2.
Total Crystal Load Capacitance C
L
= Cx1 × Cx2 / (Cx1 + Cx2).
Example
: For a Xtal C
L
of 8pF, the registers need to be programmed with X1 = X2 = 6.92 pF to get a total
C
L
= (6.92pF+ 7.5pF + 1.5pF)/2 = 7.9pF which is the closest value to 8pF.
Here, Cstray = 1.5pF; Package stray = 7.5pF
The binary settings corresponding to this value will be: X1 = X2 = “10000”.
Table 28. RAM1 – 0x12: Crystal X1 Load Capacitor Register
Bits
Default Value
Name
Function
D7
0
xtal_load_cap_x1[5:0]
Add 6.92pF load capacitance to X1.
D6
0
Add 3.46pF load capacitance to X1.
D5
0
Add 1.73pF load capacitance to X1.
D4
0
Add 0.86pF load capacitance to X1.
D3
0
Add 0.43pF load capacitance to X1.
D2
0
Add 0.43pF load capacitance to X1.
D1
0
SM1:0]
Switch Mode
Factory reserved.
Use D1 D0 = 00.
D0
1
R
F
G
M
R
S
X
2
X
1
Xtal O scillator
Cs
1
C i
1
Cs
2
C i
2
Ce
1
Ce
2
