PC87373 datasheet(127/174 Pages) NSC

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10.0 Game Port (GMP) (Continued)

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A simple push-button mechanism is typically used to implement the game device buttons. The status of each button is

sensed by the Game Port via the JOYnBTN0,1 pins as either high or low, and reflected by the GMPST register. It is the

responsibility of the software to determine the actual status of the buttons according to their polarity, which depends on the

specific implementation of the system and the game device.

10.2.4

Game Port Operation

The Game Port monitors the game device position and button status indicators by polling their status at a specific moment

via the Game Port Status register (GMPST, see Section 10.3.2).

The process of reading the position status of the game device(s) is initiated by performing a write access to the GMPST

responding bit in the GMPST register is set to 1. To capture the position indicated by the game device, the software should

poll the GMPST register and measure the time it takes for the JOYnX,Y to go high. This measurement should be performed

by measuring the time during which an axis bit is 1.

Reading the status of the buttons of the game device is done by polling the GMPST register and looking for changes in the

bits reflecting the status of the JOYnBTN0,1 pins.

Optional debouncers for the button status pins can be enabled by setting the DBNC_EN bit of the Game Port Configuration

register to 1. The Game Port Configuration register is located at Index F0h in the Game Port’s logical device configuration

space (see Section 3.16.2 on page 70). These debouncers can be used to ensure that the status of a button pin is reflected

by the associated GMPST bit only after that pin has been stable for at least 16 ms. If the debouncers are not enabled (de-

fault), it is the responsibility of the software to implement the debounce for the button status pins, if necessary.

PC87373 Datasheet(PDF) 127 Page - National Semiconductor (TI)