PSD4235G2 datasheet(36/89 Pages) STMICROELECTRONICS

36 / 89 page

PSD4235G2

36/89

Figure 15. CPLD Output Macrocell

Product Term Allocator. The CPLD has a Prod-

uct Term Allocator. PSDsoft Express, uses the

Product Term Allocator to borrow and place prod-

uct terms from one Macrocell to another. The fol-

lowing 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, which 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 Express performs this expan-

sion as needed.

Loading and Reading the Output Macrocells

(OMC). The Output Macrocells (OMC) block oc-

cupies a memory location in the MCU address

space, as defined by the CSIOP (see the section

entitled “I/O Ports”, on page 50). The flip-flops in

each of the 16 Output Macrocells (OMC) can be

loaded from the data bus by a MCU. Loading the

Output Macrocells (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 ap-

plications as loadable counters and shift registers,

mailboxes, and handshaking protocols.

Data is loaded to the Output Macrocells (OMC) on

the trailing edge of Write Strobe (WR/WRL,

CNTL0).

The OMC Mask Register. There is one Mask

Macrocells (OMC). The Mask Registers can be

used to block the loading of data to individual Out-

put 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 associated Output Macrocells

ALLOCATOR

MASK

REG.

PT CLK

CLKIN

FEEDBACK (.FB)

PORT INPUT

MUX

POLARITY

SELECT

CLR

DIN

COMB/REG

SELECT

PORT

DRIVER

INPUT

MACROCELL

I/O PIN

INTERNAL DATA BUS

DIRECTION

CLEAR (.RE)

PROGRAMMABLE

FF (D / T/JK /SR)

ENABLE (.OE)

PRESET(.PR)

MACROCELL CS

AI04946

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