Jameco Electronics Rabbit 3000 - Manual (Page 22)

User’s Manual 13
2.2.5 Parallel I/O
There are 56 parallel input/output lines divided among seven 8-bit ports designated A
through G. Most of the port lines have alternate functions, such as serial data or chip select
strobes. Parallel Ports D, E, F, and G have the capability of timer-synchronized outputs.
The output registers are cascaded as shown in Figure 2-1.
Figure 2-1. Cascaded Output Registers for Parallel Ports D and E
Stores to the port are loaded in the first-level register. That register in turn is transferred to
the output register on a selected timer clock. The clock can be selected to be the output of
Timer A1, B1, B2 or the peripheral clock (divided by 2?). The timer signal can also cause
an interrupt that can be used to set up the next bit to be output on the next timer pulse. This
feature can be used to generate precisely controlled pulses whose edges are positioned
with high accuracy in time. Applications include communications signaling, pulse width
modulation and driving stepper motors. (A separate pulse width modulation facility is also
included in the Rabbit 3000.)
Figure 2-2. Digital Filtering Input Pins
Input pins to the parallel ports are filtered by cascaded D flip flops as shown in Figure 2-2.
This prevents pulses shorter then the peripheral clock from being recognized, synchro-
nizes external pulses to the internal clock, and avoids problems with meta stability (tem-
porarily indeterminate logical conditions due to marginal set up time with respect to the
clock).
Timer Clock
Load Clock
Load Data Output Port
D Q D Q
peripheral
External Input Filtered Input
clock