COP8782C datasheet(7/28 Pages) NSC

7 / 28 page

Functional Description (Continued)

TABLE II Crystal Oscillator Configuration TA e 25 C

CKI Freq

Conditions

(kX)(MX)

(pF)

(MHz)

30 – 36

VCC e 5V

30 – 36

VCC e 5V

TABLE III RC Oscillator Configuration TA e 25 C

CKI Freq

Instr Cycle

Conditions

(kX)

(pF)

(MHz)

(ms)

22 to 27

37 to 46

VCC e 5V

100

11 to 13

74 to 90

VCC e 5V

100

09 to 11

88 to 108

VCC e 5V

Note

(RC Oscillator Configuration) 3k s R s 200k 50 pF s C s 200 pF

C RC Oscillator

CKI can be configured as a single pin RC controlled oscilla-

tor In RC oscillator mode G7 is available as a general pur-

pose input andor HALT restart control

Table III shows the variation in the oscillator frequencies as

functions of the component (R and C) values

HALT MODE

The device supports a power saving mode of operation

HALT The controller is placed in the HALT mode by setting

the G7 data bit alternatively the user can stop the clock

input (Stopping the clock input will draw more current than

setting the G7 data bit) In the HALT mode all internal proc-

essor activities including the clock oscillator are stopped

The fully static architecture freezes the state of the control-

ler and retains all information until continuing In the HALT

mode power requirements are minimal as it draws only

leakage currents and output current The applied voltage

(VCC) may be decreased down to Vr (minimum RAM reten-

tion voltage) without altering the state of the machine

There are two ways to exit the HALT mode via the RESET

or by the G7 pin A low on the RESET line reinitializes the

microcontroller and starts execution from address 0000H In

external and RC oscillator modes a low to high transition on

the G7 pin also causes the microcontroller to come out of

the HALT mode Execution resumes at the address follow-

ing the HALT instruction Except for the G7 data bit which

gets reset all RAM locations retain the values they had prior

to execution of the ‘‘HALT’’ instruction It is required that the

first instruction following the ‘‘HALT’’ instruction be a

‘‘NOP’’ in order to synchronize the clock

INTERRUPTS

The device has a sophisticated interrupt structure to allow

easy interface to the real world There are three possible

interrupt sources as shown below

A maskable interrupt on external G0 input (positive or nega-

tive edge sensitive under software control)

A maskable interrupt on timer underflow or timer capture

A non-maskable softwareerror interrupt on opcode zero

INTERRUPT CONTROL

The GIE (global interrupt enable) bit enables the interrupt

function This is used in conjunction with ENI and ENTI to

select one or both of the interrupt sources This bit is reset

when interrupt is acknowledged

ENI and ENTI bits select external and timer interrupts re-

spectively Thus the user can select either or both sources

to interrupt the microcontroller when GIE is enabled

IEDG selects the external interrupt edge (0 e rising edge

1 e falling edge) The user can get an interrupt on both

rising and falling edges by toggling the state of IEDG bit

after each interrupt

IPND and TPND bits signal which interrupt is pending After

an interrupt is acknowledged the user can check these two

bits to determine which interrupt is pending This permits the

interrupts to be prioritized under software The pending flags

have to be cleared by the user Setting the GIE bit high

inside the interrupt subroutine allows nested interrupts

The software interrupt does not reset the GIE bit This

means that the controller can be interrupted by other inter-

rupt sources while servicing the software interrupt

INTERRUPT PROCESSING

The interrupt once acknowledged pushes the program

counter (PC) onto the stack and the stack pointer (SP) is

decremented twice The Global Interrupt Enable (GIE) bit is

reset to disable further interrupts The microcontroller then

vectors to the address 00FFH and resumes execution from

that address This process takes 7 cycles to complete At

the end of the interrupt subroutine any of the following

three instructions return the processor back to the main pro-

gram RET RETSK or RETI Either one of the three instruc-

tions will pop the stack into the program counter (PC) The

stack pointer is then incremented twice The RETI instruc-

tion additionally sets the GIE bit to re-enable further inter-

rupts

Any of the three instructions can be used to return from a

hardware interrupt subroutine

The RETSK instruction

should be used when returning from a software interrupt

subroutine to avoid entering an infinite loop

Note

There is always the possiblity of an interrupt occurring during an in-

struction which is attempting to reset the GIE bit or any other interrupt

enable bit If this occurs when a single cycle instruction is being used

to reset the interrupt enable bit the interrupt enable bit will be reset

but an interrupt may still occur This is because interrupt processing

is started at the same time as the interrupt bit is being reset To avoid

this scenario the user should always use a two three or four cycle

instruction to reset interrupt enable bits

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