Rainbow Electronics AT90S8515 - Manual (Page 32)

32
AT90S8515
0841G–09/01
Timer/Counters
The AT90S8515 provides two general-purpose Timer/Counters – one 8-bit T/C and one
16-bit T/C. The Timer/Counters have individual prescaling selection from the same 10-
bit prescaling timer. Both Timer/Counters can either be used as a timer with an internal
clock time base or as a counter with an external pin connection that triggers the
counting.
Timer/Counter Prescaler Figure 28 shows the general Timer/Counter prescaler.
Figure 28. Timer/Counter Prescaler
The four different prescaled selections are: CK/8, CK/64, CK/256 and CK/1024, where
CK is the oscillator clock. For the two Timer/Counters, added selections such as CK,
external source and stop can be selected as clock sources.
8-bit Timer/Counter0 Figure 29 shows the block diagram for Timer/Counter0.
The 8-bit Timer/Counter0 can select clock source from CK, prescaled CK or an external
pin. In addition, it can be stopped as described in the specification for the
Timer/Counter0 Control Register (TCCR0). The overflow status flag is found in the
Timer/Counter Insterrupt Flag Register (TIFR). Control signals are found in the
Timer/Counter0 Control Register (TCCR0). The interrupt enable/disable settings for
Timer/Counter0 are found in the Timer/Counter Interrupt Mask Register (TIMSK).
When Timer/Counter0 is externally clocked, the external signal is synchronized with the
oscillator frequency of the CPU. To assure proper sampling of the external clock, the
minimum time between two external clock transitions must be at least one internal CPU
clock period. The external clock signal is sampled on the rising edge of the internal CPU
clock.
The 8-bit Timer/Counter0 features both a high-resolution and a high-accuracy usage
with the lower prescaling opportunities. Similarly, the high prescaling opportunities make
the Timer/Counter0 useful for lower speed functions or exact timing functions with infre-
quent actions.
TCK1 TCK0