IA6805E2
29 August 2007
Microprocessor Unit
As of Production Version 00
Timer:
The MPU contains a single 8-bit software programmable counter driven by a 7-bit software
programmable prescaler. The counter may be loaded under program control and decrements
to zero. When the counter decrements to zero, the timer interrupt request bit in the timer
control register (TCR7) is set. Figure 13 shows a block diagram of the timer. If the timer
mask bit (TCR6) and the interrupt mask bit (I) of the condition code register are cleared, an
interrupt request is generated. After completion of the current instruction, the current state
of the machine is pushed onto the stack. The timer interrupt vector address is then fetched
from locations $1FF8 and $1FF9 and the interrupt routine is executed, unless the MPU was
in the WAIT mode in which case the interrupt vector address in locations $1FF6 and $1FF7
is fetched. Power-On-Reset causes the counter to set to $FF.
NOTE: 1. Prescaler and counter are clocked on the falling edge of the internal
clock (AS) or external input.
2. Counter is written to during Data Strobe (DS) and counts down continuously.
TIMER
(PIN 37)
TIMER_n
EXT
CLK
PRESCALER
(7 BITS)
COUNTER
(8 BITS)
INTERRUPT
CONTROL
2 - TO - 1
MUX
INTERRUPT
READ WRITE
INT
CLK
ENABLE /
DISABLE_n
INTERNAL_n / EXTERNAL
INTERNAL
CLOCK
TCR4 TCR5
TCR3 TCR2 TCR1 TCR0
SETTING TCR3 CLEARS
PRESCALER TO ÷ 1
SOFTWARE FUNCTIONS
Figure 13. Timer Block Diagram
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IA211081401-03
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