Atari 400 Hardware Manual

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 joy-
stick 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.
Paddles: 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. Turn~ng 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 fron 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 scaq 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 Bi: 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).
Keyboard Controllers: 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~ 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 +SV 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 (O=button is pressed, 1=not pressed).
II.31

Atari 400 Specifications

General

Manufacturer	Atari
Model	400
Type	Home Computer
Release Year	1979
CPU	MOS Technology 6502
CPU Speed	1.79 MHz
ROM	10 KB
Operating System	Atari OS
Keyboard	Membrane keyboard
RAM	8 KB (expandable to 48 KB)
Graphics	ANTIC and GTIA chips
Sound	4 channels
Display	RF output for connection to TV
Storage	Optional cassette tape drive or floppy disk drive
Ports	Cartridge
Display Resolution	320x192 (16 colors)