PSD833F2 datasheet(54/128 Pages) STMICROELECTRONICS

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14.5

Product Term Allocator

The CPLD has a Product Term Allocator. The PSDabel compiler uses the Product Term

Allocator to borrow and place product terms from one macrocell to another. The following list

summarizes how product terms are allocated:

●

McellAB0-McellAB7 all have three native product terms and may borrow up to six more

●

McellBC0-McellBC3 all have four native product terms and may borrow up to five more

●

McellBC4-McellBC7 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 already 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 macrocells (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 expansion as needed.

14.6

Loading and reading the Output macrocells (OMC)

The Output macrocells (OMC) block occupies a memory location in the MCU address

space, as defined by the CSIOP block (see Section 16: I/O ports). 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 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 Write Strobe

(WR, CNTL0) (edge loading) or during the time that Write Strobe (WR, CNTL0) is active

(level loading). The method of loading is specified in PSDsoft Express Configuration.

14.7

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 associated Output macrocells (OMC). For example, suppose McellAB0-

McellAB3 are being used for a state machine. You would not want a MCU write to McellAB

to overwrite the state machine registers. Therefore, you would want to load the Mask

14.8

The Output Enable of the OMC

The Output macrocells (OMC) block can be connected to an I/O port pin as a PLD output.

The output 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 enabled upon Power-up if no

output enable equation is defined and if the pin is declared as a PLD output in PSDsoft

Express.

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