Issue 147 October 2002
Watch Me Pull A Rabbit Out of My Hat

Start • Rabbit 3000 • No-Risk CISC • Peck-O-Periphs • Time Traveler • One-Stop Shopping • Keep Motoring • Sources and PDF

TIME TRAVELER

The Rabbit 3000 has a lot of features in the time domain, starting with the clock pins. Make that clocks, because the chip provides connections for separate low- (32.768 kHz) and high-speed (up to 30 MHz) inputs. An on-chip doubler boosts the clock rate to the 50-MHz-plus maximum specification.

The low-speed input (i.e., watch crystal) drives the real-time clock (implemented as a 48-bit counter with dedicated battery backup pin) and watchdog timer while the high-speed clock drives the processor and peripherals.

The low-speed clock can also drive the processor and peripherals, which is the basis for the low-power Sleepy mode that can throttle the CPU down to almost 2 kHz (i.e., 32.768 kHz divided by 16). Slowing the clock to reduce CPU power consumption isn’t a new concept, but Rabbit shows attention to detail with short and self-timed chip-select options that minimize memory chip power consumption as well.

EMI reduction is a hot topic that I predict will only get hotter. The Rabbit 3000 is one of the first micros to incorporate spread-spectrum clocking as an option. When enabled, pseudo-random jitter is automatically injected into the clock, spreading the radiated noise (see Figure 2).

Figure 2—Three spread-spectrum settings (i.e., off, normal, and strong) are available. Spreading interference around isn’t the same as reducing it, which makes the technique a bit controversial, though it no doubt facilitates meeting FCC regulations.

Application timing tasks are well served by a plethora of clocks, counters, and pins. In this case, a picture is easily worth the thousand words it would take to describe all of the options (see Figure 3). It’s definitely a sophisticated and high-resolution setup compared to run-of-the-mill 8-bit micros.

Figure 3—Besides handling the mundane chores such as serial data rates, the Rabbit 3000 timer subsystem goes well beyond the typical MCU’s reload timer with features like high-speed input capture, PWM, and quadrature decode.

A registered output option means even parallel output gets precise. You can configure output port updates to trigger off timers for superior timing precision without the CPU having to babysit the port in software.