PC87366 datasheet(157/240 Pages) NSC

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9.0 Game Port (GMP) (Continued)

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When a position counter of a game device stops counting, its associated Position Counter Ready bit in the GMPXST register

is set. All the software has to do in this case is wait until the counters associated with the game device are ready, and then

read their values. The least significant byte of a position counter should be read first. The full, 16-bit count value should be

calculated as follows:

X/Y Position Count = GMPnX/YL + (GMPnX

/YH * 256)

where:

GMPnX/YL indicates the low byte of the position counter of device n (either X or Y axis)

GMPnY/HL indicates the high byte of the position coutner of device n (either X or Y axis)

It is the responsibility of the software to calibrate itself according to the actual count values acquired when the game device

was set to indicate its extreme horizontal and vertical positions.

If a position counter has reached the full count of FFFFh, it means that this counter has overflowed. Overflow means that

the counter has reached its full count before the corresponding axis indicator has reached the level of logic 1. In such a case,

the software must decide what to do.

The Game Port supports the following clock frequencies for operating the position counters:

1 MHz clock (default)

500 KHz clock.

The clock frequency for the position counter of each game device is configured via the GMPCTL register and should be set

by the software to match the physical components of the external game device interface circuitry. Setting the desired position

counter frequency should be done before initiating a position status reading process.

Reading the status of the buttons of the game device(s) is done the same way it is done in Legacy mode. In addition, an

optional debouncer of 16 msec is implemented on each button status input. The debouncers are disabled by default and

may be enabled by software via the GMPCTL register.

9.2.5

Operation Control

When the Game Port is operated in Legacy mode, it can only be operated by polling (see Section 9.2.4,

Legacy Mode).

When the Game Port is operated in Enhanced mode, both kinds of status reading operations (position and button) can be

performed using polling or interrupt controlled operation.

If polling controlled operation is preferred, the software should poll either the GMPLST register for the direct status of the

buttons as in Legacy mode, or the GMPXST register which provides indications regarding button events detected by hard-

ware. The GMPXST register should also be polled for the status of the position counters. When the status is ready, the

counter values can be read. These values reflect the positions indicated by the game device.

If interrupt controlled operation is preferred, the software should first define the events on which an interrupt request is to be

issued. This is done by writing the required values to the GMPEPOL (see Section 9.3.14) and GMPIEN (see Section 9.3.5)

registers. The GMPEPOL register defines the events on which the buttons cause an interrupt request to be issued. These

events are all edge-triggered. The GMPIEN register determines what events are physically routed to the interrupt request

assigned to the Game Port. An independent interrupt enable bit is implemented in the GMPIEN register for each one of the

four buttons and two position counters of the two supported game devices.

PC87366 Datenblatt(PDF) 157 Page - National Semiconductor (TI)

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