Wakeup Timer - Philips LPC2119 User Manual

Philips Semiconductors Preliminary User Manual
ARM-based Microcontroller LPC2119/2129/2292/2294

WAKEUP TIMER

The purpose of the wakeup timer is to ensure that the oscillator and other analog functions required for chip operation are fully
functional before the processor is allowed to execute instructions. This is important at power on, all types of Reset, and whenever
any of the aforementioned functions are turned off for any reason. Since the oscillator and other functions are turned off during
Power Down mode, any wakeup of the processor from Power Down mode makes use of the Wakeup Timer.
The Wakeup Timer monitors the crystal oscillator as the means of checking whether it is safe to begin code execution. When
power is applied to the chip, or some event caused the chip to exit Power down mode, some time is required for the oscillator to
produce a signal of sufficient amplitude to drive the clock logic. The amount of time depends on many factors, including the rate
of Vdd ramp (in the case of power on), the type of crystal and its electrical characteristics (if a quartz crystal is used), as well as
any other external circuitry (e.g. capacitors), and the characteristics of the oscillator itself under the existing ambient conditions.
Once a clock is detected, the Wakeup Timer counts 4096 clocks, then enables the Flash memory to initialize. When the Flash
memory initialization is complete, the processor is released to execute instructions if the external Reset has been de-asserted.
In the case where an external clock source is used in the system (as opposed to a crystal connected to the oscillator pins), the
possibility that there could be little or no delay for oscillator start-up must be considered. The Wakeup Timer design then ensures
that any other required chip functions will be operational prior to the beginning of program execution.
The LPC2119/2129/2292/2294 does not contain any analog function such as comparators that operate without clocks or any
independent clock source such as a dedicated Watchdog oscillator. The only remaining functions that can operate in the absence
of a clock source are the external interrupts (EINT0, EINT1, EINT2 and EINT3) and the CAN controllers. When an external
interrupt is enabled for wakrup, and its selected event occurs, an oscillator wakeup cycle is started. Similarly,if a CAN block is
enabled for wakeup and activity occurs on its CAN bus, an oscillator wakeup cycle is statrted. The actual interrupt (if any) occurs
after the wakeup time expires, and is handled by the Vectored Interrupt Controller (VIC).
However, the pin multiplexing on the LPC2119/2129/2292/229 (see Pin Configuration on page 86 and Pin Connect Block on page
100) was designed to allow other peripherals to, in effect, bring the device out of power down mode. The following pin-function
2
pairings allow interrupts from events relating to UART0 or 1, SPI 0 or 1, or the I C: RxD0 / EINT0, SDA / EINT1, SSEL0 / EINT2,
RxD1 / EINT3, DCD1 / EINT1, RI1 / EINT2, SSEL1 / EINT3.
To put the device in power down mode and allow activity on one or more of these buses or lines to power it back up, software
should reprogram the pin function to External Interrupt, select the appropriate mode and polarity for the Interrupt, and then select
power down mode. Upon wakeup software should restore the pin mulitplexing to the peripheral function.
All of the bus- or line-activity indications in the list above happen to be low-active. If software wants the device to come out of
power -down mode in response to actity on more than one pin that share the same EINTi channel, it should program low-level
sensitivity for that channel, because only in level mode will the channel logically OR the signals to wake the device.
The only flaw in this scheme is that the time to restart the oscillator prevents the LPC2119/2129/2292/229 from capturing the bus
or line activity that wakes it up. Idle mode is more appropriate than power-down mode for devices that must capture and respond
to external activity in a timely manner.
To summarize: on the LPC2119/2129/2292/2294, the Wakeup Timer enforces a minimum reset duration based on the crystal
oscillator, and is activated whenever there is a wakeup from Power Down mode or any type of Reset.
System Control Block 69 January 08, 2004