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CY7C63310, CY7C638xx
Document 38-08035 Rev. *K Page 28 of 83
11.1 Power on Reset
POR occurs every time the power to the device is switched on.
POR is released when the supply is typically 2.6V for the upward
supply transition, with typically 50 mV of hysteresis during the
power on transient. Bit 4 of the System Status and Control
Register (CPU_SCR) is set to record this event (the register
contents are set to 00010000 by the POR). After a POR, the
microprocessor is held off for approximately 20 ms for the V
CC
supply to stabilize before executing the first instruction at
address 0x00 in the Flash. If the V
CC
voltage drops below the
POR downward supply trip point, POR is reasserted. The V
CC
supply must ramp linearly from 0 to 4V in less than 200 ms.
Note The PORS status bit is set at POR and is cleared only by
the user. It cannot be set by firmware.
11.2 Watchdog Timer Reset
The user has the option to enable the WDT. The WDT is enabled
by clearing the PORS bit. After the PORS bit is cleared, the WDT
cannot be disabled. The only exception to this is if a POR event
takes place, which disables the WDT.
The sleep timer is used to generate the sleep time period and the
Watchdog time period. The sleep timer uses the Internal 32 kHz
Low power Oscillator system clock to produce the sleep time
period. The user can program the sleep time period using the
Sleep Timer bits of the OSC_CR0 Register (Table 10-4 on page
23). When the sleep time elapses (sleep timer overflows), an
interrupt to the Sleep Timer Interrupt Vector is generated.
The Watchdog Timer period is automatically set to be three
counts of the Sleep Timer overflow. This represents between two
and three sleep intervals depending on the count in the Sleep
Timer at the previous WDT clear. When this timer reaches three,
a WDR is generated.
The user can either clear the WDT, or the WDT and the Sleep
Timer. When the user writes to the Reset WDT Register
(RES_WDT), the WDT is cleared. If the data that is written is the
hex value 0x38, the Sleep Timer is also cleared at the same time.
12. Sleep Mode
The CPU is put to sleep only by the firmware. This is
accomplished by setting the Sleep bit in the System Status and
Control Register (CPU_SCR). This stops the CPU from
executing instructions, and the CPU remains asleep until an
interrupt comes pending, or there is a reset event (either a Power
on Reset, or a Watchdog Timer Reset).
The Low Voltage Detection circuit (LVD) drops into fully
functional power reduced states, and the latency for the LVD is
increased. The actual latency is traded against power
consumption by changing Sleep Duty Cycle field of the ECO_TR
Register.
The Internal 32 kHz Low speed Oscillator remains running.
Before entering the suspend mode, the firmware can optionally
configure the 32 kHz Low speed Oscillator to operate in a low
power mode to help reduce the over all power consumption
(Using Bit 7, Table 10-2 on page 22). This helps save
approximately 5 μA; however, the trade off is that the 32 kHz Low
speed Oscillator is less accurate.
All interrupts remain active. Only the occurrence of an interrupt
wakes the part from sleep. The Stop bit in the System Status and
Control Register (CPU_SCR) must be cleared for a part to
resume out of sleep. The Global Interrupt Enable bit of the CPU
Flags Register (CPU_F) does not have any effect. Any
unmasked interrupt wakes the system up. As a result, any
interrupts not intended for waking must be disabled through the
Interrupt Mask Registers.
When the CPU enters sleep mode the CPUCLK Select (Bit 1,
Table 10-3 on page 22) is forced to the Internal Oscillator. The
internal oscillator recovery time is three clock cycles of the
Internal 32 kHz Low power Oscillator. The Internal 24 MHz
Oscillator restarts immediately on exiting Sleep mode. If an
external clock is used, firmware switches the clock source for the
CPU.
On exiting sleep mode, after the clock is stable and the delay
time has expired, the instruction immediately following the sleep
instruction is executed before the interrupt service routine (if
enabled).
The Sleep interrupt allows the microcontroller to wake up
periodically and poll system components while maintaining very
low average power consumption. The Sleep interrupt may also
be used to provide periodic interrupts during non-sleep modes.
Table 11-2. Reset Watchdog Timer (RESWDT) [0xE3] [W]
Bit # 7 6 5 4 3 2 1 0
Field Reset Watchdog Timer [7:0]
Read/Write W W W W WWW W
Default 0 0 0 0 000 0
Any write to this register clears Watchdog Timer; a write of 0x38 also clears the Sleep Timer.
Bit [7:0]: Reset Watchdog Timer [7:0]
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