EM6821 datasheet(10/68 Pages) EMMICRO

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EM6821

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4.4 Digital Watchdog Timer Reset

The digital watchdog is a simple, non-programmable, 2-bit timer, that counts on each rising edge of Ck[1]. It will

generate a system reset if it is not periodically cleared. The watchdog timer function can be inhibited by

activating an inhibit digital watchdog bit (NoLogicWD) located in RegVldCntl. At power up, and after any

system reset, the watchdog timer is activated.

If for any reason the CPU stops, then the watchdog timer can detect this situation and activate the system reset

signal. This function can be used to detect program overrun, endless loops, etc. For normal operation, the

watchdog timer must be reset periodically by software at least every 2.5 seconds (system clock = 32 KHz), or a

system reset signal is generated.

The watchdog timer is reset by writing a ‘1’ to the WDReset bit in the timer. This resets the timer to zero and

timer operation restarts immediately. When a ‘0’ is written to WDReset there is no effect. The watchdog timer

operates also in the standby mode and thus, to avoid a system reset, one should not remain in standby mode

for more than 2.5 seconds.

From a system reset state, the watchdog timer will become active after 3.5 seconds. However, if the watchdog

timer is influenced from other sources (i.e. prescaler reset), then it could become active after just 2.5 seconds.

It is therefore recommended to use the Prescaler IRQHz1 interrupt to periodically reset the watchdog every

second.

It is possible to read the current status of the watchdog timer in RegSysCntl2. After watchdog reset, the

counting sequence is (on each rising edge of CK[1]) : ‘00’, ‘01’, ‘10’, ‘11’ {WDVal1 WDVal0}. When going into

the ‘11’ state, the watchdog reset will be active within ½ second. The watchdog reset activates the system

reset which in turn resets the watchdog. If the watchdog is inhibited it’s timer is reset and therefore always

reads ‘0’.

Table 4.4.1 Watchdog Timer Register RegSysCntl2

Bit

Name

Reset

R/W

Description

WDReset

R/W

Reset the Watchdog

1 -> Resets the Logic Watchdog

0 -> No action

The Read value is always '0'

SleepEn

R/W

See Operating modes (sleep)

WDVal1

Watchdog timer data Ck[1] divided by 4

WDVal0

Watchdog timer data Ck[1] divided by 2

4.5 CPU State after Reset

Reset initializes the CPU as shown in Table 4.5.1 below.

Table 4.5.1 Initial CPU Value after Reset.

Name

Bits

Symbol

Initial Value

Program counter 0

PC0

hex 000 (as a result of Jump 0)

Program counter 1

PC1

Undefined

Program counter 2

PC2

Undefined

Stack pointer

PSP[0] selected

Index register

Undefined

Carry flag

Undefined

Zero flag

Undefined

Halt

HALT

Instruction register

Jump 0

Periphery registers

Reg.

See peripheral memory map

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