Clocking
September 5, 2002
Document #: 38-12010 CY Rev. ** CMS Rev. 3.20
37
External Crystal Oscillator Trim Register (ECO_TR, Address = Bank 1, EBh)
7.1.4
External Crystal Oscillator Component Connections and Selections
Crystal – 32.768 kHz watch crystal such as EPSON
C-002RX (12.5 pF load capacitance)
Capacitors – C1, C2
Use NPO-type ceramic caps
C1 = C2 = 25 pF - (Package Cap) - (Board Parasitic
Cap)
Note
: Use this equation if you do not employ PLL mode.
If you do employ PLL with the External Crystal Oscillator,
see Application Note AN2027 under Support at http://
www.cypressmicro.com for equation and details. An
error of 1 pF in C1 and C2 gives about 3 ppm error in fre-
quency.
Table 37:
External Crystal Oscillator Trim Register
Bit #
7
6
5
4
3
2
1
0
POR
FS1
1.
FS = Factory set trim value
FS1
0
0
FS1
FS1
FS1
FS1
Read/Write
W
W
--
--
W
W
W
W
Bit Name
PSSDC [1] PSSDC [0]
Reserved
Reserved
Amp [1]
Amp [0]
Bias [1]
Bias [0]
Bit [7:6]
: PSSDC [1:0] Power System Sleep Duty Cycle. (Not recommended for customer alteration)
0 0 = 1/128
0 1 = 1/512
1 0 = 1/32
1 1 = 1/8
Bit 5
: Reserved
Bit 4
: Reserved
Bit [3:2]
: Amp [1:0] Sets the amplitude of the adjustment. (Not recommended for customer alteration)
Bit [1:0]
: Bias [1:0] Sets the bias of the adjustment. (Not recommended for customer alteration)
Figure 8: External Crystal Oscillator Connections
XtalIn
XtalOut
Crystal
Crystal
Crystal
Crystal
C1
C2
Vcc
Vcc
Table 38:
Typical Package Capacitances
Package
Package Capacitance
8 PDIP
0.9 pF
20 PDIP
2 pF
20 SOIC
1 pF
20 SSOP
0.5 pF
28 PDIP
2 pF
28 SOIC
1 pF
28 SSOP
0.5 pF
44 TQFP
0.5 pF
48 PDIP
5 pF
48 SSOP
0.6 pF
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