A variety of controllers can be plugged into the four jacks on the front of the console. This includes joysticks, paddle (pot), twelve-key keyboard, and light pen (when available).
The controller ports are read through the PORTA and PORTB registers and the POT and TRIG registers. The OS reads these registers during vertical blank and stores into its own RAM locations. These are STICK, PADDL0 through PADDL7, PTRIG’S and STRIG'S. The OS sets up PORTA AND PORTB for input. This is done by setting PACTL or PORTB (Port Control) bit 2 to a 0 (to select the direction control register), then writing all 0's to the desired port. PACTL (PBCTL) bit 2 is then changed back to a 1, allowing the program to read from the port. The ports can also be set up for output by writing 1's instead of 0 s while the direction control mode is selected.
The joysticks have four switches, one each for right (R), left (L), back (B) and forward (P).
These switches are read through PORTA. and PORTB. A fifth switch is activated by pressing the red trigger button. The trigger buttons are read from TRIGO through TRIG3. A value of 0 indicates that a button has been pressed and a 1 indicates that it has not been pressed.
The TRIG registers are normally read directly, but they can be used in a latched mode. Writing a zero to bit 2 of GRACTL disables the latches and sets them to 1. Writing a 1 to bit 2 enables the latches. If a joystick trigger button is pushed at any time while bit 2 of GRACTL is 1 the latch value will change to zero and stay that way. A program can use this to determine whether the joystick trigger buttons have ever been pressed during a certain period of time.
The paddles come in pairs, so eight paddles can be connected to the four jacks. The paddles are read by storing into POTGO, then reading the POT registers at least 228 lines later. The values range from 0 (with the paddle turned to the right) to 228 (paddle turned counter-clockwise).
The value indicates how many TV lines it takes to charge up the capacitor which is the series with the potentiometer. Turning the knob to the right lowers the resistance, so the capacitor charges up quickly. Turning the knob to the left increases the resistance and the charging time.
The capacitor dump transistors are used to discharge the capacitors so that a new reading can be mode. The POTGO command clears the counters and turns off the dump transistors to allow the capacitors to charge up. The ALLPOT register contains one bit for each paddle. When the capacitor has charged up to the threshold value the ALLPOT bit changes from one to zero and the POT register contains the correct readings. Bit 2 of SKCTL (Serial Port Control) enables fast pot scan. In this mode, It takes only two scan lines to charge up the capacitors to the maximum level instead of 228 lines. Bit 2 is first set to 0 to dump the capacitors. Then Bit 2 is set to 1 to start the pot scan. The fast pot scan is not as accurate as the normal scan mode.
Bit 2 of SKCTL must be set to 0 to use normal scan mode. Otherwise, the capacitors will never dump. Note that some paddles have a range smaller than 0 to 228 due to differences in the pots. The left and right paddle triggers for each paddle pair are read from the left and right bits for the corresponding joystick (PORTA or PORTB).
Each keyboard controller has a twelve-key pad and plugs into a joystick controller port. The first step in using the keyboard is to select a row by setting the port direction to output and writing a 0 to the bit in the PORTA or PORTB register which selects the desired row (see PORTA, SECTION III). The other rows should have 1's written to them. Columns are read through the POT and TRIG registers (see controller PORT PINOUT chart in section III). Appendix H of the BASIC Reference Manual contains a Basic program which reads the controllers. The first and second columns of the keyboard use the same pins as the pots for the paddle controllers, so they are read by reading the POT (or PADDL) registers. When a button is pushed, the pot line is grounded, so the pot capacitors never charge up to the threshold level and the reading is 228 (the maximum). When the button in the selected row and column is not pushed the capacitor is connected to +5V through a relatively small resistor, giving a POT value of about 2 (this may vary). Since the reading is not critical, the fast pot scan mode can be used, so that only a 2 line wait is required between selecting the row and reading the POT register. The convention has been adopted of comparing the POT reading with 10 (decimal). If is it greater than 10 then the button has been pressed. The third column is read through the joystick trigger line, so it works just like a joystick trigger (0=button is pressed, 1=not pressed).
A light pen is a device that can detect the electron beam as it sweeps across the TV screen. It is used to point directly at an image on the TV display. Applications include selecting menu items and drawing lines. The ATARI 400/800 hardware was designed so that a light pen can be plugged into any of the joystick controller ports (see end of section III).
When any one of the joystick trigger lines (pin 6) is pulled low, the ANTIC chip takes the current VCOUNT value and stores it in PENV. The horizontal color clock value (0-227 decimal) is stored in PENH. The least significant bit is inaccurate and should be ignored. Since there are a number of delays involved in displaying the data and changing the light pen register, each system must be calibrated. Software which uses the light pen should contain a user-interactive calibration routine. For example, the user could point the light pen at a crosshair in the centre of the screen and the program could compute the required horizontal offset. PENH will wrap around from 227 to 0 near the right hand edge of a standard width display because of the delay. The pen will not work if it is pointed at a black area of the screen, since the electron beam is turned off. It is a good idea to read two (or more) values and average them, since the user will probably not hold the pen perfectly steady.