PSD835G2 datasheet(44/102 Pages) STMICROELECTRONICS

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PSD835G2

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Product Term Allocator

The CPLD has a Product Term Allocator. The PSD

uses the Product Term Allocator to borrow and

place product terms from one macrocell to anoth-

er. The following list summarizes how product

terms are allocated:

–

McellA0-McellA7 all have three native product

terms and may borrow up to six more

–

McellB0-McellB3 all have four native product

terms and may borrow up to five more

–

McellB4-McellB7 all have four native product

terms and may borrow up to six more.

Each macrocell may only borrow product terms

from certain other macrocells. Product terms al-

ready in use by one macrocell are not available for

another macrocell.

If an equation requires more product terms than

are available to it, then “external” product terms

are required that consume other Output Macro-

cells (OMC). If external product terms are used,

extra delay is added for the equation that required

the extra product terms.

This is called product term expansion. PSDsoft

performs this expansion as needed.

Loading and Reading the Output Macrocells

(OMC)

The Output Macrocells (OMC) block occupies a

memory location in the MCU address space, as

defined

the

CSIOP

block

(see

I/O

PORTS, page 59). The flip-flops in each of the 16

Output Macrocells (OMC) can be loaded from the

data bus by a MCU. Loading the Output Macro-

cells (OMC) with data from the MCU takes priority

over internal functions. As such, the preset, clear,

and clock inputs to the flip-flop can be overridden

by the MCU. The ability to load the flip-flops and

read them back is useful in such applications as

loadable counters and shift registers, mailboxes,

and handshaking protocols.

Data can be loaded to the Output Macrocells

(OMC) on the trailing edge of the Write Strobe

(WR, CNTL0) signal.

The OMC Mask Register

There is one Mask Register for each of the two

groups of eight Output Macrocells (OMC). The

Mask Registers can be used to block the loading

of data to individual Output Macrocells (OMC).

The default value for the Mask Registers is 00h,

which allows loading of the Output Macrocells

(OMC). When a given bit in a Mask Register is set

to a '1,' the MCU is blocked from writing to the as-

sociated Output Macrocells (OMC). For example,

suppose McellA0-McellA3 are being used for a

state machine. You would not want a MCU WRITE

to McellA to overwrite the state machine registers.

Therefore, you would want to load the Mask Reg-

ister for McellA (Mask Macrocell AB) with the value

0Fh.

The Output Enable of the OMC

The Output Macrocells (OMC) block can be con-

nected to an I/O port pin as a PLD output. The out-

put enable of each port pin driver is controlled by

a single product term from the AND Array, ORed

with the Direction Register output. The pin is en-

abled upon Power-up if no output enable equation

is defined and if the pin is declared as a PLD out-

put in PSDsoft.

If the Output Macrocell (OMC) output is declared

as an internal node and not as a port pin output in

the PSDabel file, the port pin can be used for other

I/O functions. The internal node feedback can be

routed as an input to the AND Array.

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