SX User’s Manual Rev. 3.1 151 © 2000 Scenix Semiconductor, Inc. All rights reserved. www.scenix.com Chapter 6 Timers and Interrupts 6.1     Introduction The SX core has two different timers: the Real-Time Clock/Counter (RTCC) and the Watchdog timer.
The RTCC timer can be used to keep track of elapsed time or to count external events. The Watchdog
timer provides an automatic escape route from infinite loops and other program errors. An RTCC timer
overflow triggers an interrupt, whereas a Watchdog timer overflow triggers a device reset. Some  SX  devices  such  as  the  SX48/52BD  offer  additional  timers.  These  non-core  features  are
described in separate chapters later in this manual. The SX device supports interrupts from the RTCC circuit and from eight Multi-Input Wakeup pins in
Port B. An interrupt causes a jump to the bottom of the program memory (address 0000h), where the
interrupt  service  routine  is  located.  The  service  routine  is  terminated  by  an  RETI  or  RETIW
instruction, which  causes the  device to jump back to the point in the  program where the interrupt
occurred and restore the program context at that point. 6.2 Real-Time Clock/Counter The Real-Time Clock/Counter is a general-purpose timer that can be used to keep track of elapsed time
or to keep a count of pulses received on the RTCC input pin. The RTCC register is a memory-mapped,
8-bit register that can keep a count up to 256. An 8-bit prescaler register can be used to extend the
maximum count to 65,536. Figure 6-1  is  a  block  diagram  showing  the  RTCC  circuit,  including  the  RTCC  register,  the  8-bit prescaler   register,   the   Watchdog   timer   (WDT)   register,   and   the   supporting   multiplexers   and
configuration bits that control the RTCC timer. The  RTCC  register  is  clocked  (incremented)  either  by  the  internal  instruction  clock  or  by  pulses
received on the RTCC input pin. The choice is controlled by the RTS bit in the OPTION register. Set
this bit to 1 to count pulses on the RTCC input pin, or clear this bit to 0 to count instruction cycles. If you select the RTCC input pin as the clock source, the RTE_ES bit in the OPTION register specifies
the type of transition sensed on the pin. Clear the bit to 0 to sense rising edges (low-to-high transitions)
or set the bit to 1 to sense falling edges (high-to-low transitions) on the RTCC pin.