PSoC CY8CTMG20x and CY8CTST200 TRM, Document No. 001-53603 Rev. *C
69
9. External Crystal Oscillator (ECO)
This chapter briefly explains the External Crystal Oscillator (ECO) and its associated registers. The 32.768 kHz external crys-
tal oscillator circuit allows the user to replace the internal low speed oscillator with a more precise time source. For a quick ref-
erence of all PSoC registers in address order, refer to the
Register Reference chapter on page 187
.
9.1
Architectural Description
The External Crystal Oscillator (ECO) circuit requires only
the following external components: an inexpensive watch
crystal and two small value capacitors. The XTALIn (P2[3])
and XTALOut (P2[5]) pins connect to a 32.768 kHz watch
crystal and the two external capacitors bypass these pins to
ground.
shows the external connections needed
to implement the ECO. See the
for information on enabling the ECO. Transitions
between the internal and external oscillator domains may
produce glitches on the clock bus.
During the process of activating the ECO, there must be a
hold-off period before using it as the 32.768 kHz source.
This hold-off period is partially implemented in hardware
using the sleep timer. Firmware must set up a sleep period
of one second (maximum ECO settling time), and then
enable the ECO in the OSC_CR0 register. At the one sec-
ond timeout (the sleep interrupt), the switch is made by
hardware to the ECO. If the ECO is subsequently deacti-
vated, the Internal Low Speed Oscillator (ILO) will again be
activated and the switch is made back to the ILO immedi-
ately.
The ECO Exists bit (ECO EX, bit 0 of ECO_CONFIG) is
used to control whether the switch-over is allowed or locked.
This is a write once bit. It is written early in code execution
after a Power on Reset (POR) or external reset (XRES)
event. A '1' in this bit indicates to the hardware that a crystal
exists in the system, and firmware is allowed to switch back
and forth between ECO and ILO operation. If the bit is '0',
switch-over to the ECO is locked out.
Figure 9-1. External Components for the ECO
The ECO Exists Written bit (ECO EXW, bit 1 of
ECO_CONFIG) is read only and is set on the first write to
this register. When this bit is '1', it indicates that the state of
ECO EX is locked. This is illustrated in
.
P2[5]
P2[3]
Vss
Vss
Vss
X1
C1
C2
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