PC87393F datasheet(85/148 Pages) NSC

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5.0 Game Port (GMP) (Continued)

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The varying resistors RVX and RVY are usually implemented in the game device. Their resistance values are determined

directly by the horizontal and vertical positions, respectively, indicated by the game device. The waveform shaping circuits

are usually implemented outside the game device using constant resistors (RCX/Y) and constant capacitors (CX/Y). Together

with RVX/Y, these components implement two R-C structures, the varying parameters of which are used to determine the

exact momentary position indicated by the game device.

When the Game Port is enabled and not in the midst of a game device position reading process, it drives the JOYnX,Y pins

low. In this state, the capacitors CX/Y are completely discharged.

5.2.2

Capturing the Position

The process of capturing the position indicated by the game device is initiated by a command given to the Game Port to

release the JOYnX,Y lines, thus allowing the capacitors CX/Y to be charged. This command is given by performing a write

access to offset 1 from the Game Port base address, which is the offset of the Game Port Legacy Status Register (GMPLST,

see Section 5.3.3). Once JOYnX,Y pins are released, RCX/Y and RVX/Y start charging CX/Y, and the voltage level of the

JOYnX,Y pins increases until it reaches VIH. This process is described in Figure 15.

Figure 15. Position Reading Process Waveform (not drawn to scale)

The vertical and horizontal positions indicated by the game device are determined by measuring the time it takes for the

voltage level on the JOYnX,Y pins to reach the level of logic 1. Since the charging time is determined by the resistance val-

ues of RVX/Y, measuring this time actually indicates the resistance values of RVX/Y, and therefore also reflects the position

indicated by the game device.

Once an axis pin is sensed as logic 1, the axis circuit discharge control is activated in order to discharge CX/Y. This causes

the corresponding axis pin to be driven low for approximately 1.5

µsec. After that, this axis line is held low until another po-

sition reading process is initiated.

During the charge time and the 1.5

µsec discharge time which follows, the corresponding axis line does not respond to any

reading process initiation. This prevents software from disturbing the position reading process and makes the position read-

ing processes of all axis lines independent of each other.

5.2.3

Button Status Indication

The button(s) status indication mechanism is described in Figure 16. Although this figure shows an active-low button

(RBU0>>RBD0), the polarity of the button can be either high or low, assuming that the Game Port software is aware of the

button’s polarity.

Figure 16. Game Device Button Status Indication Mechanism

Time

VCX/Y [V]

VDD

VIH

Time measured as

position indication

Discharge

Charge

Idle

Drive

Low

Release

Drive

Low

RBU0

Game

Port

JOYnBTN0

JOYnBTN1

Game Device Button Status Circuit

Pins

Buttons 0,1

Status

Indicators

Input Path

RBD0

PC87393F Datasheet(PDF) 85 Page - National Semiconductor (TI)

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