HCTL-2000 Datasheet, PDF (11/18 Pages) Agilent(Hewlett-Packard) – Quadrature Decoder/Counter Interface ICs

CLK
CHA_FILT*
CHBFILT*
U/D
CNTDCDR*
INTERNAL COUNTER
ROLL OVER
FF 00
START INHIBIT
CNTCAS
COUNT
FFFDH
FFFEH
FFFFH
0000H
FFFFH
FFFDH
*CHAFILT AND CHBFILT ARE THE OUTPUTS
OF THE DIGITAL NOISE FILTER (SEE FIGURES 7 AND 8).
Figure 12. Decode and Cascade Output Diagram.
Cascade Considerations
(HCTL-2020 Only)
The HCTL-2020’s cascading
system allows for position reads
of more than two bytes. These
reads can be accomplished by
latching all of the bytes and then
reading the bytes sequentially
over the 8-bit bus. It is assumed
here that, externally, a counter
followed by a latch is used to
count any count that exceeds 16
bits. This configuration is
compatible with the HCTL-2020
internal counter/latch
combination.
Consider the sequence of events
for a read cycle that starts as the
HCTL-2020’s internal counter
rolls over. On the rising clock
edge, count data is updated in the
internal counter, rolling it over. A
count-cascade pulse (CNT_CAS)
will be generated with some delay
after the rising clock edge (t_CHD).
There will be additional
propagation delays through the
external counters and registers.
Meanwhile, with SEL and OE low
to start the read, the internal
latches are inhibited at the falling
edge and do not update again till
the inhibit is reset. If the CNTCAS
pulse now toggles the external
counter and this count gets
latched a major count error will
occur. The count error is because
the external latches get updated
when the internal latch is
inhibited.
Valid data can be ensured by
latching the external counter data
when the high byte read is started
(SEL and OE low). This latched
external byte corresponds to the
count in the inhibited internal
latch. The cascade pulse that
occurs during the clock cycle
when the read begins gets
counted by the external counter
and is not lost.
For example, suppose the HCTL-
2020 count is at FFFFH and an
external counter is at F0H, with
the count going up. A count
occurring in the HCTL-2020 will
cause the counter to roll over and
a cascade pulse will be generated.
A read starting on this clock cycle
will show FFFFH from the HCTL-
2020. The external latch should
read F0H, but if the host latches
the count after the cascade signal
propagates through, the external
latch will read F1H.
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