COP87L88CL Datasheet, PDF(17/34 Page) National Semiconductor (TI) – 8-Bit One-Time Programmable OTP Microcontroller

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COP87L88CL Datasheet, PDF (17/34 Pages) National Semiconductor (TI) – 8-Bit One-Time Programmable OTP Microcontroller

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Interrupts (Continued)

SOFTWARE TRAP

The Software Trap (ST) is a special kind of non-maskable

interrupt which occurs when the INTR instruction (used to

acknowledge interrupts) is fetched from ROM and placed

inside the instruction register This may happen when the

PC is pointing beyond the available ROM address space or

when the stack is over-popped

When an ST occurs the user can re-initialize the stack

pointer and do a recovery procedure (similar to reset but

not necessarily containing all of the same initialization pro-

cedures) before restarting

The occurrence of an ST is latched into the ST pending bit

The GIE bit is not affected and the ST pending bit (not

accessible by the user) is used to inhibit other interrupts

and to direct the program to the ST service routine with the

VIS instruction The RPND instruction is used to clear the

software interrupt pending bit This bit is also cleared on

reset

The ST has the highest rank among all interrupts

Nothing (except another ST) can interrupt an ST being

serviced

WATCHDOG

The device contains a WATCHDOG and clock monitor The

WATCHDOG is designed to detect the user program getting

stuck in infinite loops resulting in loss of program control or

âârunawayââ programs The Clock Monitor is used to detect

the absence of a clock or a very slow clock below a speci-

fied 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 val-

ue 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 III shows the WDSVR register

TABLE III WATCHDOG Service Register (WDSVR)

Window

Select

Key Data

Clock

Monitor

01100

54321

The lower limit of the service window is fixed at 2048 in-

struction cycles Bits 7 and 6 of the WDSVR register allow

the user to pick an upper limit of the service window

Table IV shows the four possible combinations of lower and

upper limits for the WATCHDOG service window This flexi-

bility 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 IV WATCHDOG Service Window Select

WDSVR

Bit 7

WDSVR

Bit 6

Service Window

(Lower-Upper Limits)

2k â 8k tc Cycles

2k â 16k tc Cycles

2k â 32k tc Cycles

2k â 64k tc Cycles

Clock Monitor

The Clock Monitor aboard the device can be selected or

deselected under program control The Clock Monitor is

guaranteed not to reject the clock if the instruction cycle

clock (1 tc) is greater or equal to 10 kHz This equates to a

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

WATCHDOG Operation

The WATCHDOG and Clock Monitor are disabled during

reset The device comes out of reset with the WATCHDOG

armed the WATCHDOG Window Select (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

the WATCHDOG service window (ii) enabling or disabling of

the Clock Monitor Hence the first write to WDSVR Register

involves selecting or deselecting the Clock Monitor select

the WATCHDOG service window and match the

WATCHDOG key data Subsequent writes to the WDSVR

the WATCHDOG service window value and the key data

(bits 7 through 1) in the WDSVR Register Table V shows

the sequence of events that can occur

The user must service the WATCHDOG at least once be-

fore the upper limit of the serivce window expires The

WATCHDOG 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

between 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 The WDOUT pin is in the high impedance state in the

inactive state Upon triggering the WATCHDOG the logic

will pull the WDOUT (G1) pin low for an additional

16 tc â 32 tc cycles after the signal level on WDOUT pin goes

below the lower Schmitt trigger threshold After this delay

the device will stop forcing the WDOUT output low

The WATCHDOG service window will restart when the

WDOUT pin goes high It is recommended that the user tie

the WDOUT pin back to VCC through a resistor in order to

pull WDOUT high

A WATCHDOG service while the WDOUT signal is active

will be ignored The state of the WDOUT pin is not guaran-

teed on reset but if it powers up low then the WATCHDOG

will time out and WDOUT will enter high impedance state

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