PC87303 datasheet(90/114 Pages) NSC

90 / 114 page

90 Keyboard Controller and Real-Time Clock (Continued)

Timer Interrupt

If the timer interrupt is enabled upon the timer overflow an

interrupt occurs It causes the program to perform a subrou-

tine call to program address 007h The interrupt is cleared

by this subroutine call The current Program Counter (PC)

and the upper 4 bits of the PSW is pushed into the stack

before the call occurs At the end of the timer interrupt serv-

ice routine an RETR instruction restores the PSW and the

PC Because the timer interrupt has a lower priority than the

IBF interrupt simultaneous IBF and timer interrupts cause

the timer interrupt to be pending It is served as soon as the

program returns from the IBF interrupt service routine

Input Buffer Full (IBF) Interrupt

If the lBF interrupt is enabled when there is a host write

operation to the keyboard controller (CS e 0 IOW e 0) an

interrupt occurs The processor saves the current Program

Counter and the upper 4 bits of the PSW into the stack and

performs a subroutine call to the program address 003h

Upon entering the interrupt service routine further interrupts

are held off The interrupt is re-enabled upon the execution

of the RETR instruction

917 Oscillator and Instruction Timing

The oscillator pins of the keyboard controller are X1 and X2

when bit 7 of KRR is 0 When bit 7 of KRR is 1 the Key-

board Controller clock is SYSCLK

Figures 9-10 and 9-11

show the connections for internal and external clock config-

uration via X1 and X2 pins

The oscillator clock is divided by 3 to generate the state

timing then is further divided down by 5 to generate the

instruction timing

(Figure 9-12) Hence each instruction cy-

cle consists of 5 states and 15 clock cycles

Most of the keyboard controller instructions require only one

instruction cycle The others require two cycles Refer to the

instruction set for details

TLC12074 – 45

FIGURE 9-10 Internal Clock Connection

TLC12074 – 46

FIGURE 9-11 External Clock Connection

TLC12074 – 47

FIGURE 9-12 Instruction Cycle Timing

PC87303 Datasheet(PDF) 90 Page - National Semiconductor (TI)

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