CY7C63722C
CY7C63723C
CY7C63743C
FOR
FOR
Document #: 38-08022 Rev. *C
Page 11 of 49
before the part executes code. See Section 10.1 for more
details.
1 = Disables the LVR circuit.
0 = Enables the LVR circuit.
Bit 2: Precision USB Clocking Enable
The Precision USB Clocking Enable only affects operation
in internal oscillator mode. In that mode, this bit must be
set to 1 to cause the internal clock to automatically pre-
cisely tune to USB timing requirements (6 MHz ±1.5%).
The frequency may have a looser initial tolerance at pow-
er-up, but all USB transmissions from the chip will meet the
USB specification.
1 = Enabled. The internal clock accuracy is 6 MHz ±1.5%
after USB traffic is received.
0 = Disabled. The internal clock accuracy is 6 MHz ±5%.
Bit 1: Internal Clock Output Disable
The Internal Clock Output Disable is used to keep the inter-
nal clock from driving out to the XTALOUT pin. This bit has
no effect in the external oscillator mode.
1 = Disable internal clock output. XTALOUT pin will drive
HIGH.
0 = Enable the internal clock output. The internal clock is
driven out to the XTALOUT pin.
Bit 0: External Oscillator Enable
At power-up, the chip operates from the internal clock by
default. Setting the External Oscillator Enable bit HIGH dis-
ables the internal clock, and halts the part while the external
resonator/crystal oscillator is started. Clearing this bit has
no immediate effect, although the state of this bit is used
when waking out of suspend mode to select between inter-
nal and external clock. In internal clock mode, XTALIN pin
will be configured as an input with a weak pull-down and
can be used as a GPIO input (P2.1).
1 = Enable the external oscillator. The clock is switched to
external clock mode, as described in Section 9.1.
0 = Enable the internal oscillator.
9.1
Internal/External Oscillator Operation
The internal oscillator provides an operating clock, factory set
to a nominal frequency of 6 MHz. This clock requires no
external components. At power-up, the chip operates from the
internal clock. In this mode, the internal clock is buffered and
driven to the XTALOUT pin by default, and the state of the
XTALIN pin can be read at Port 2.1. While the internal clock is
enabled, its output can be disabled at the XTALOUT pin by
setting the Internal Clock Output Disable bit of the Clock
Configuration Register.
Setting the External Oscillator Enable bit of the Clock Config-
uration Register HIGH disables the internal clock, and halts
the part while the external resonator/crystal oscillator is
started. The steps involved in switching from Internal to
External Clock mode are as follows:
1. At reset, chip begins operation using the internal clock.
2. Firmware sets Bit 0 of the Clock Configuration Register. For
example,
mov A, 1h
; Set Bit 0 HIGH (External Oscil-
lator Enable bit). Bit 7 cleared
gives faster start-up
iowr F8h
; Write to Clock Configuration
Register
3. Internal clocking is halted, the internal oscillator is disabled,
and the external clock oscillator is enabled.
4. After the external clock becomes stable, chip clocks are
re-enabled using the external clock signal. (Note that the
time for the external clock to become stable depends on the
external resonating device; see next section.)
5. After an additional delay the CPU is released to run. This
delay depends on the state of the Ext. Clock Resume Delay
bit of the Clock Configuration Register. The time is 128
µs
if the bit is 0, or 4 ms if the bit is 1.
6. Once the chip has been set to external oscillator, it can only
return to internal clock when waking from suspend mode.
Clearing bit 0 of the Clock Configuration Register will not
re-enable internal clock mode until suspend mode is
entered. See Section 11.0 for more details on suspend
mode operation.
If the Internal Clock is enabled, the XTALIN pin can serve as
a general purpose input, and its state can be read at Port 2,
Bit 1 (P2.1). Refer to Figure 12-8 for the Port 2 Data Register.
In this mode, there is a weak pull-down at the XTALIN pin. This
input cannot provide an interrupt source to the CPU.
9.2
External Oscillator
The user can connect a low-cost ceramic resonator or an
external oscillator to the XTALIN/XTALOUT pins to provide a
precise reference frequency for the chip clock, as shown in
Figure 9-1. The external components required are a ceramic
resonator or crystal and any associated capacitors. To run
from the external resonator, the External Oscillator Enable bit
of the Clock Configuration Register must be set to 1, as
explained in the previous section.
Start-up times for the external oscillator depend on the
resonating device. Ceramic resonator based oscillators
typically start in less than 100
µs, while crystal based oscil-
lators take longer, typically 1 to 10 ms. Board capacitance
should be minimized on the XTALIN and XTALOUT pins by
keeping the traces as short as possible.
An external 6-MHz clock can be applied to the XTALIN pin if
the XTALOUT pin is left open.
10.0
Reset
The USB Controller supports three types of resets. The effects
of the reset are listed below. The reset types are:
1. Low-voltage Reset (LVR)
2. Brown Out Reset (BOR)
3. Watchdog Reset (WDR)
The occurrence of a reset is recorded in the Processor Status
and Control Register (Figure 20-1). Bits 4 (Low-voltage or
Brown-out Reset bit) and 6 (Watchdog Reset bit) are used to
record the occurrence of LVR/BOR and WDR respectively.
The firmware can interrogate these bits to determine the cause
of a reset.
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