CDP1855 View Datasheet(PDF) - Intersil
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CDP1855 Datasheet PDF : 15 Pages
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CDP1855, CDP1855C
A second division is performed using the remainder from the
first division (in Y) as the more significant 8N-bits of the divi-
dend and the less significant half of the original dividend
loaded into the Z register. The divisor in X remains unaltered
and is, by definition, larger than the remainder from the first
division which is in Y. The resulting value in Z becomes the
less significant 8N-bits of the final quotient and the value in Y
is, as usual, the remainder.
Extending this technique to more steps allows division of any
size number by an 8N-bit divisor.
Note that division by zero is never permitted and must be
tested for and handled in software.
The following example illustrates the use of this algorithm.
Example:
Assume three MDU's capable of a by 24-bit division. The
problem is to divide 00F273, 491C06H by 0003B4H.
Step 1: 000000 , 00F273 / 0003B4 = 000041 R=0001BF
Y
Z(MS)
X
Z1
Y1
Step 2: 0001BF , 491C06 / 0003B4 = 78C936 R=00000E
Y1
Z(LS)
X
Z2
Y2
Result: 000041 , 78C936
R=00000E
Z1
Z2
Y2
The Z register can simply be reset using bit 2 of the control
word and another divide can be done in order to further
divide the remainder.
3. Multiply Operation
For a multiply operation the two numbers to be multiplied are
loaded in the X and Z registers. The result is in the Y and Z
register with Y being the more significant half and Z the less
significant half. The X register will be unchanged after the
operation is completed.
The original contents of the Y register are added to the
product of X and Z. Bit 3 of the control word will reset
register Y to 0 if desired.
Functional Description of
CDP1855 Terminals
CE - Chip Enable (Input):
A high on this pin enables the CDP1855 MDU to respond to
the select lines. All cascaded MDU's must be enabled
together. CE also controls the three-state C.O./O.F., output
of the most significant MDU.
Clear (Input):
The CDP1855 MDU(s) must be cleared upon power-on with
a low-on this pin. The clear signal resets the sequence
counters, the shift pulse generator, and bits 0 and 1 of the
control register.
CTL - Control (Input):
This is an input pin. All CTL pins must be wired together and
to the YL of the most significant CDP1855 MDU and to the
ZR of the least significant CDP1855 MDU. This signal is
used to indicate whether the registers are to be operated on
or only shifted.
C.O./O.F. - Carry Out/Over Flow (Output):
This is a three-state output pin. It is the CDP1855 Carry Out
signal and is connected to Cl (CARRY-IN) of the next more
significant CDP1855 MDU, except for on the most significant
MDU. On that MDU it is an overflow indicator and is enabled
when chip enables is true. A low on this pin indicates that an
overflow has occurred. The overflow signal is latched each
time the control register is loaded, but is only meaningful
after a divide command.
YL, YR - Y-Left, Y-Right:
These are three-state bi-directional pins for data transfer
between the Y registers of cascaded CDP1855 MDU's. The
YR pin is an output and YL is an input during a multiply and
the reverse is true at all other times. The YL pin must be
connected to the YR pin of the next more significant MDU.
An exception is that the YL pin of the most significant
CDP1855 MDU must be connected to the ZR pin of the least
significant MDU and to the CTL pins of all MDU's. Also the
YR pin of the least significant MDU is tied to the ZL pin of the
most significant MDU.
ZL, ZR - Z-Left, Z-Right:
These are three-state bi-directional pins for data transfers
between the “Z” registers of cascaded MDU's. The ZR pin is
an output and ZL is an input during a multiply and the
reverse is true at all other times. The ZL pin must be tied to
the YR pin of the next more significant MDU. An exception is
that the ZL in of the most significant MDU must be con-
nected to the YR pin of the least significant MDU. Also, the
ZR pin of the least significant MDU is tied to the YL of the
most significant MDU.
Shift - Shift Clock:
This is a three-state bi-directional pin. It is an output on the
most significant MDU. And an input on all other MDU's. It
provides the MDU system timing pulses. All SHIFT pins must
be connected together for cascaded operation. A maximum
of the 8N +1 shifts are required for an operation where "N"
equals the number of MDU devices that are cascaded.
CLK - Clock (Input):
This pin should be grounded on all but the most significant
MDU. There is an optional reduction of clock frequency avail-
able on this pin if so desired, controlled by bit 7 of the control
byte.
STB - Strobe (Input):
When RD/WE is low, data is latched from bus lines on the
falling edge of this signal. It may be asynchronous to the
clock. Strobe also increments the selected register's
sequence counter during reads and writes. TPB would be
used in CDP1800 systems.
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