DanBoris' Tech Blog - Vertical Ball Pt II

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The ball vertical motion is controlled by a slip counter just like the horizontal.

 

 

The counter is clocked by the /HSYNC signal so it will increment once per line. Since /VBLANK goes to the ENT input of B3 it will stop the count during VBLANK. When the two stages of the counter reach 255 the load signal will be triggered by B2. The upper stage of the counter is loaded with 0, and the lower portion is loaded with the output of the vertical control circuit. The values from the vertical control result in the following numbers of counts:

 

7 – 248

8 – 247

9 – 246

10 – 245

11 – 244

12 – 243

13 – 242

 

There are 245 visible lines on the screen (excluding the VBLANK region), so a value of 10 from the vertical control will result in no vertical motion, a value less then 10 will move the ball down on the screen, and a value greater then 10 will move the ball up on the screen.

 

When the counter reaches a value of 252 the output of E2 will go low and stay low until the counter resets 4 counts later, making the ball 4 pixels high.

 

 

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