COP8SA datasheet(34/60 Pages) NSC

34 / 60 page

10.0 WATCHDOG/Clock Monitor

The devices contain a user selectable WATCHDOG and

clock monitor. The following section is applicable only if

WATCHDOG feature has been selected in the ECON regis-

ter. The WATCHDOG is designed to detect the user program

getting stuck in infinite loops resulting in loss of program con-

trol or “runaway” programs.

The WATCHDOG logic contains two separate service win-

dows. While the user programmable upper window selects

the WATCHDOG service time, the lower window provides

protection against an infinite program loop that contains the

WATCHDOG service instruction.

The COP8SAx devices provide the added feature of a soft-

ware trap that provides protection against stack overpops

and addressing locations outside valid user program space.

The Clock Monitor is used to detect the absence of a clock or

a very slow clock below a specified rate on the CKI pin.

The WATCHDOG consists of two independent logic blocks:

WD UPPER and WD LOWER. WD UPPER establishes the

upper limit on the service window and WD LOWER defines

the lower limit of the service window.

Servicing the WATCHDOG consists of writing a specific

value to a WATCHDOG Service Register named WDSVR

which is memory mapped in the RAM. This value is com-

posed of three fields, consisting of a 2-bit Window Select, a

5-bit Key Data field, and the 1-bit Clock Monitor Select field.

Table 6 shows the WDSVR register.

TABLE 6. WATCHDOG Service Register (WDSVR)

Window

Key Data

Clock

Select

Monitor

01100

The lower limit of the service window is fixed at 256 instruc-

tion cycles. Bits 7 and 6 of the WDSVR register allow the

user to pick an upper limit of the service window.

Table 7 shows the four possible combinations of lower and

upper limits for the WATCHDOG service window. This flex-

ibility in choosing the WATCHDOG service window prevents

any undue burden on the user software.

Bits 5, 4, 3, 2 and 1 of the WDSVR register represent the

5-bit Key Data field. The key data is fixed at 01100. Bit 0 of

the WDSVR Register is the Clock Monitor Select bit.

TABLE 7. WATCHDOG Service Window Select

WDSVR

Clock

Service Window

Bit 7

Bit 6

Monitor

(Lower-Upper Limits)

2048–8k t

C Cycles

2048–16k t

C Cycles

2048–32k t

C Cycles

2048–64k t

C Cycles

Clock Monitor Disabled

Clock Monitor Enabled

10.1 CLOCK MONITOR

The Clock Monitor aboard the device can be selected or de-

selected under program control. The Clock Monitor is guar-

anteed not to reject the clock if the instruction cycle clock (1/

C) is greater or equal to 10 kHz. This equates to a clock

input rate on CKI of greater or equal to 100 kHz.

10.2 WATCHDOG/CLOCK MONITOR OPERATION

The WATCHDOG is enabled by bit 2 of the ECON register.

When this ECON bit is 0, the WATCHDOG is enabled and

pin G1 becomes the WATCHDOG output with a weak pullup.

The WATCHDOG and Clock Monitor are disabled during re-

set. The device comes out of reset with the WATCHDOG

armed, the WATCHDOG Window Select bits (bits 6, 7 of the

WDSVR Register) set, and the Clock Monitor bit (bit 0 of the

WDSVR Register) enabled. Thus, a Clock Monitor error will

occur after coming out of reset, if the instruction cycle clock

frequency has not reached a minimum specified value, in-

cluding the case where the oscillator fails to start.

The WDSVR register can be written to only once after reset

and the key data (bits 5 through 1 of the WDSVR Register)

must match to be a valid write. This write to the WDSVR reg-

ister involves two irrevocable choices: (i) the selection of the

WATCHDOG service window (ii) enabling or disabling of the

Clock Monitor. Hence, the first write to WDSVR Register in-

volves selecting or deselecting the Clock Monitor, select the

WATCHDOG service window and match the WATCHDOG

key data. Subsequent writes to the WDSVR register will

compare the value being written by the user to the WATCH-

DOG service window value and the key data (bits 7 through

1) in the WDSVR Register.

Table 8 shows the sequence of

events that can occur.

The user must service the WATCHDOG at least once before

the upper limit of the service window expires. The WATCH-

DOG may not be serviced more than once in every lower

limit of the service window. The user may service the

WATCHDOG as many times as wished in the time period be-

tween the lower and upper limits of the service window. The

first write to the WDSVR Register is also counted as a

WATCHDOG service.

The WATCHDOG has an output pin associated with it. This

is the WDOUT pin, on pin 1 of the port G. WDOUT is active

low and must be externally connected to the RESET pin or to

some other external logic which handles WATCHDOG event.

The WDOUT pin has a weak pullup in the inactive state. This

pull-up is sufficient to serve as the connection to V

CC for sys-

tems which use the internal Power On Reset. Upon trigger-

ing the WATCHDOG, the logic will pull the WDOUT (G1) pin

low for an additional 16 t

C–32 tC cycles after the signal level

on WDOUT pin goes below the lower Schmitt trigger thresh-

old. After this delay, the device will stop forcing the WDOUT

output low. The WATCHDOG service window will restart

when the WDOUT pin goes high.

A WATCHDOG service while the WDOUT signal is active will

be ignored. The state of the WDOUT pin is not guaranteed

on reset, but if it powers up low then the WATCHDOG will

time out and WDOUT will go high.

The Clock Monitor forces the G1 pin low upon detecting a

clock frequency error. The Clock Monitor error will continue

until the clock frequency has reached the minimum specified

value, after which the G1 output will go high following 16

C–32 tC clock cycles. The Clock Monitor generates a con-

tinual Clock Monitor error if the oscillator fails to start, or fails

to reach the minimum specified frequency. The specification

for the Clock Monitor is as follows:

1/t

C > 10 kHz — No clock rejection.

1/t

C < 10 Hz — Guaranteed clock rejection.

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