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CS89712

High-Performance, Low-Power System-on-Chip with 10BASE-T Ethernet Controller

Cirrus Logic 

CS89712

(i.e., the main batteries are low), corresponding to

a low level on nPWRFL or BATOK.

From the Standby State, if the WAKEUP signal is

applied with no clock except the 32 kHz clock run-

ning, the CS89712 will be initialized into a state

where it is ready to start and is waiting for the CPU

to start receiving its clock. The CPU will still be

held in reset at this point. After the first clock is ap-

plied, there will be a delay of about eight clock cy-

cles before the CPU is enabled. This delay allows

the CPU clock to settle.

2.2.1.1 UART in Standby State

During the Standby State, the UARTs are disabled

and cannot detect any activity (i.e., start bit) on the

receiver. If this functionality is required then this

can be accomplished in software by the following

method:

1) Permanently connect the RX pin to one of the

active low external interrupt pins.

2) Ensure that on entry to the Standby State, the

chosen interrupt source is not masked, and the

UART is enabled.

3) Send a preamble that consists of one start bit,

8 bits of zero, and one stop bit. This will cause

the CS89712 to wake and execute the enabled

interrupt vector.

The UART will automatically be re-enabled when

the processor re-enters the Operating State, and the

preamble will be received. Since the UART was

not awake at the start of the preamble, the timing of

the sample point will be off-center during the pre-

amble byte. However, the next byte transmitted

will be correctly aligned. Thus, the actual first real

byte to be received by the UART will be correct.

2.2.2 Idle State

If in the Operating State, the Idle State can be en-

tered by writing to a special internal memory loca-

tion (HALT) in the CS89712. If an interrupt occurs,

the CS89712 will return immediately back to the

Operating State and execute the next instruction.

The WAKEUP signal can not be used to exit the

Idle State. It is only used to exit the Standby State.

In the Idle State, the device functions just like it

does when in the Operating State. However, the

CPU clock is halted while it waits for an event such

as a key press to generate an interrupt. The PLL al-

ways remains active in the Idle State.

2.2.3 Keyboard Interrupt Wakeup

For the case of the keyboard interrupt, the follow-

ing options are available and are selectable accord-

ing to bits 1 and 3 of the SYSCON2 register (refer

to Section 3.5.2 for register details).

• If the KBWEN bit (SYSCON2 bit 3) is set low,

then a keypress will cause a transition from a

power saving state only if the keyboard inter-

rupt is non-masked (i.e., the interrupt mask reg-

ister 2 (INTMR2 bit 0) is high).

• When KBWEN is high, a keypress will cause

the device to wake up regardless of the state of

the interrupt mask register. This is called the

“Keyboard Direct Wakeup’ mode. In this

mode, the interrupt request may not get ser-

viced. If the interrupt is masked (i.e., the inter-

rupt mask register 2 (INTMR2 bit 0) is low),

the processor simply starts re-executing code

from where it left off before it entered the pow-

er saving state. If the interrupt is non-masked,

then the processor will service the interrupt.

• When the KBD6 bit (SYSCON2 bit 1) is low,

all 8 Port A inputs are OR’ed together to pro-

duce the internal wakeup signal and keyboard

interrupt request. This is the default reset state.

• When the KBD6 bit (SYSCON2 bit 1) is high,

only the lowest 6 bits of Port A are OR’ed to-

gether to produce the internal wakeup signal

and keyboard interrupt request. The two most

significant bits of Port A are available as GPIO

when this bit is set high.

DS502PP2

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