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Apple II
Technical Notes
                                                  Developer Technical Support

Apple IIe
#6:    The Apple II Paddle Circuits

Revised by:    Glenn A. Baxter                                  November 1988
Written by:    Peter Baum                                            May 1984

This Technical Note describes the paddle circuit used in the Apple II family 
of computers.


Since Apple has introduced machines with internal clock speeds which may not 
be exactly 1.023 MHz, it is best to use the PREAD firmware call to read paddle 
data.  This Note assumes that the clock speed of the system is exactly 1.023 
MHz.  If you want to insure accuracy in reading paddle data, you should make 
sure the system is first running at the correct speed.  Enough information is 
provided so that you can write your own PREAD routine, although this is 
discouraged.  If the program runs on an Apple IIGS or some future machine, 
your custom paddle reading routine will fail to give the correct results.

Circuit Description

The value of the Apple paddles (or joystick) is determined by a software 
timing loop reading a change of state in a timing circuit.  The paddles 
consist of a variable resistor (from 0-150k ohms) which makes up part of the 
timing circuit.  There is a routine in the monitor ROM, called PREAD, which 
counts the time until a state change occurs in the paddle circuit.  This time 
is translated into a value between 0 and 255.

The block diagrams in Figures 1 and 2 show the paddle circuit for the Apple 
][+, Apple IIc, and the Apple IIe.  The large block on the left illustrates 
part of the circuitry inside the 558 timer chip.  The 558 chip consists of 
four of these blocks, with all four paddle triggers lines shorted together on 
the motherboard and activated by the soft switch at $C070.  The outputs of the 
558 chip run into a multiplexer, which places the appropriate signal onto the 
high bit of the data bus when a paddle soft switch address in the range $C064 
$C067 is read.  The Apple IIc uses a 556 timer rather than the 558 chip and 
only supports two paddles, 0 and 1.

The 100 ohm resistor and .022 microfarad capacitor are on the motherboard, 
with the variable resistor in the paddle.  Each of the four paddle inputs have 
their own capacitor and resistor.  Since these components can vary by as much 
as five percent from Apple to Apple, this circuit is not a very exact analog 
to digital converter.  If a paddle is moved from one Apple to another without 
changing the resistance (turning the knob), the paddle read routine will 
probably calculate a different value for each machine.  About the only feature 
of the paddle read routine that a programmer can depend on is that the value 
returned will rise if the paddle resistance increases (or fall if the 
resistance decreases).

The paddle timing circuit on the Apple ][+ and Apple IIc is slightly different 
than the one on the Apple IIe.  On the Apple IIe, the 100 ohm fixed resistor 
is between the transistor and the capacitor, while the variable resistor in 
the paddle is connected directly to the capacitor.  On the Apple ][+ and IIc, 
the capacitor is connected directly to the transistor and the fixed resistor 
is in series with paddle resistor.

|                   556 Timer                   |
|    ^ VCC                                      |               _____________
|    |                                          |              |             |
|    \                                          |              |   Paddle    |
|    /                                          |              |      ^      |
| 5K \                                          |              |      |  ^   |
|    /    +-----------------------------------------O---+      |      \ /    |
|    \    |                                     |       |      |      //     |
|    |    |            ___________              |       |      |      X      |
|    +------- + |\    |           |             |       |      |     //      |
|    |    |     | >---|RESET      |             |       |      |    / \      |
|    \    +-- - |/    |           |             |       |      |______|______|
|    /                |           |             |       |             |
| 5K \  Comparator    |           |             |       |    100 ohm  |
|    /                | FLIP FLOP |         +-------O---+--+-\/\/\/\--+
|    \                |           |         |   |          |
|    |                |           |        /    |          |
|    +------- + |\    |          _|      |/     |         _|_
|    |          | >---|SET       Q|---+--|\     |         ___ .022uF
|    \    +-- - |/    |___________|   |  | \>   |          |
|    /    |                           |     |   |          |
| 5K \    |                           |     +---|---O------+
|    /    |                           |   __|__ |
|    \    |                           |    ---  |
|  __|__  |                           |     -   |
|   ---   |                           |  |\     |
|    -    |                           +--| >O---|---O Output
|         |                              |/     |     ($C06x)
          O Trigger

                    Figure 1 - Apple ][+ and IIc Paddle Circuit

An Example of a Typical Paddle Read Routine

The timing circuit works by discharging a capacitor through a transistor, then 
shutting the transistor off and letting the paddle charge the capacitor by 
supplying current through the variable resistor.  The rate at which the 
capacitor charges is a function of the variable resistance; the lower the 
paddle resistance, the greater the current and the faster the capacitor 
charges.  When the capacitor reaches a predetermined value it changes the 
state of a flip flop. The paddle read routine counts the time it takes for the 
capacitor to rise and change the flip flop.

Let's step through an example of a typical paddle read operation.  For now we 
will assume the capacitor has already been discharged and in a few pages I 
will explain when this assumption can be made and when it cannot.

The software starts by reading the soft switch at location $C070, which 
strobes the trigger lines on the 558 timer.  This action causes two events to 
occur, the output signal (which is read at $C064-$C067 for paddle 0-3, 
respectively) goes high and the transistor turns off.

The software, after initially strobing the trigger line, executes a timing 
loop which reads the state of the output signal.  When the output signal 
changes from high to low the software jumps out of the timing loop and returns 
a value indicating the time.  The monitor PREAD routine consists of a 11 Ásec. 
loop and will return a value between 0 and 255.  (Note:  The firmware listing 
is wrong and says the loop is 12 Ásec.)  The timing loop returns 255 if the 
circuit takes longer than 2.82 ms for the state change to occur.

|                   558 Timer                   |
|    ^ VCC                                      |               _____________
|    |                                          |              |             |
|    \                                          |              |   Paddle    |
|    /                                          |              |      ^      |
| 5K \                                          |              |      |  ^   |
|    /    +---------------------------------+   |              |      \ /    |
|    \    |                                 |   |              |      //     |
|    |    |            ___________          |   |              |      X      |
|    +------- + |\    |           |         |   |              |     //      |
|    |    |     | >---|RESET      |         |   |              |    / \      |
|    \    +-- - |/    |           |         |   |              |______|______|
|    /                |           |         |   |                     |
| 5K \  Comparator    |           |         |   |      100 ohm        |
|    /                | FLIP FLOP |         +-------O--\/\/\/\--+-----+
|    \                |           |         |   |               |
|    |                |           |        /    |               |
|    +------- + |\    |          _|      |/     |              _|_
|    |          | >---|SET       Q|---+--|\     |              ___ .022uF
|    \    +-- - |/    |___________|   |  | \>   |               |
|    /    |                           |     |   |               |
| 5K \    |                           |     +---|---O-----------+
|    /    |                           |   __|__ |
|    \    |                           |    ---  |
|  __|__  |                           |     -   |
|   ---   |                           |  |\     |
|    -    |                           +--| >O---|---O Output
|         |                              |/     |     ($C06x)
          O Trigger

                      Figure 2 - Apple IIe Paddle Circuit


FB21:A0 00             2  LDY  #0        ;INIT COUNTER
FB23:EA                2  NOP            ;COMPENSATE FOR 1ST COUNT
FB24:EA                2  NOP
FB28:10 04             2  BPL  RTS2D     ;BRANCH WHEN TIMED OUT
FB2A:C8                2  INY            ;INCREMENT COUNTER
FB2B:D0 F8             3  BNE  PREAD2    ;CONTINUE COUNTING
FB2D:88                   DEY            ;COUNTER OVERFLOWED
FB2E:60        RTS2D      RTS            ;RETURN W/VALUE 0-255

Inside the 558 timer chip, when the trigger is strobed low, the comparator 
that feeds the set input of the flip flop is triggered, which in turn sets the 
output of the 558 timer.  At the same time, the transistor, which has held the 
capacitor near ground by sinking current from it, is shut off.  The capacitor 
can now charge using the current supplied by the paddle.  The smaller the 
paddle's resistance, the more current the paddle will supply and the faster 
the capacitor charges.  After some time, the capacitor will charge to the 
threshold value of 3.3 volts, which is set by the voltage divider network in 
the 558 timer, and the comparator that feeds the reset input on the flip flop 
will trigger.  This trigger sets the output signal ($C06x) of the 558 timer 
low, which indicates to the software that the circuit has timed out.

              _____        ___________________________________________________
Trigger $C070      |      |

                           _|                               |<------ Threshold
Feedback to              _|                                 |_
Reset Comp _____________|                                     |_______________

                        |                                   |
Output _________________|                                   |_________________
                                    Timing Value
                                0 - 2.82 milliseconds

                      Figure 3 - Paddle Circuit Recharge Timing

Resetting the flip flop turns the transistor on, which discharges the 
capacitor very quickly (normally less than 250 ns).  That paddle can then be 
read again.

A Closer Look at the Hardware

The First Anomaly

Notice that the last sentence states that the paddle can be read again and not 
the paddles.  If another paddle is read immediately after the first, it may 
yield the wrong value.  To demonstrate this, I will step through an example of 
reading a second paddle immediately after finishing the first.

In this example I will assume that the first paddle has been set with a very 
low resistance, while the second paddle has a high resistance.  The first 
paddle will time out very quickly and return with a small value, while the 
second paddle will take longer and yield a larger value.

We start reading the paddles by testing the paddle outputs to see if they are 
low, which indicates that the capacitor has been discharged.  Assuming that 
the outputs are low, the next step is to trigger the 558 timer ($C070), which 
turns off the transistor and allows the capacitors to charge.  Since all of 
the trigger input lines are shorted together, all four of the capacitors will 
charge, but at different rates since the paddle resistances have been set to 
different values.  The voltage on the capacitor for the first paddle will 
reach the threshold voltage very quickly since the paddle resistance has been 
set low, therefore the timing loop will time out quickly.

At this point the capacitor for the second paddle is still charging and has 
not yet reached the threshold since the paddle resistance was set to a high 
value.  The transistor for the second paddle is still turned off due to the 
initial trigger used for reading paddle one.  This means that the capacitor 
for the second paddle has not been discharged.

Any attempts at reading the second paddle now will only yield false results.  
The capacitor is partly charged and therefore will reach the threshold value 
much faster than if the capacitor had been completely discharged.  If the 
timing loop is used, it will return with a smaller value than it would if the 
capacitor had been completely discharged.  Notice that retriggering (reading 
location $C070) the 558 timer will not help, since that only keeps the 
transistor turned off and does not help discharge the capacitor.  The only way 
for the capacitor to discharge is to let the circuit time out completely by 
letting the capacitor charge until it resets the flip flop.

To read the second paddle, the capacitor must first be discharged, which is 
only done when the threshold value is reached and the 558 timer flip flop is 
reset.  The only way to guarantee that the capacitor is discharged is if the 
transistor is on.  This condition is met when the paddle output is low.  
Therefore, start every paddle read either by waiting for at least 3 ms before 
strobing the trigger input or testing to make sure that the paddle output is 

If after 4 ms the paddle output is not low, then there is a good chance that 
there is no paddle connected.  This result may also indicate that a peripheral 
with a larger maximum value resistor than the 150k ohms used by the Apple 
paddles is attached. Some peripheral devices use this technique of a larger 
variable resistor so that more than 256 points of resolution can be 
determined.  Of course, this requires a custom software driver and the monitor 
PREAD routine cannot be used.

Apple IIe Anomalies

The problem with Apple IIe paddle input is that the capacitor may not be 
discharged by the transistor.  Typically, the transistor will discharge the 
capacitor in less than 250 ns on the Apple ][+.  But on the Apple IIe, if the 
paddle resistance is very low then the paddle may supply enough current to 
always keep the capacitor charged.

Because the fixed resistor (100 ohms) on the Apple IIe motherboard is between 
the capacitor and the transistor, there will be a voltage drop across the 
resistor if the capacitor stays charged.  When the transistor is shut off by 
the trigger strobe, this voltage drop will disappear and the capacitor, which 
may be near the threshold voltage, will trigger the reset comparator earlier 
than it would if the capacitor had been discharged completely.  The net affect 
of this is that the paddles will read zero on the Apple IIe when they would 
read a small value on the Apple ][+ or IIc.

Other circuits which expect the capacitor to discharge completely may not work 
properly.  A circuit which attempts to simulate a paddle through active 
components such as a digital to analog converter may be able to source enough 
current that the capacitor never discharges and the paddle always reads zero.

It should also be noted that due to electromagnetic interference, later model 
IIe computers actually have an extra capacitor attached between the BUTTON 
inputs and ground.  This essentially slows the response time of the input, 
making a fully digital input appear a bit more analog (no pun intended).  Care 
should be taken in designing system which depend on a certain repetition rate 
of the button inputs.  Careful engineering and testing across systems should 
prevent any problems.  As an example, adding a transistor output stage to 
drive the button inputs to the appropriate states might be a good idea for a 
serializing A/D.  A joystick would not require this kind of circuit because 
the user input is too slow to be affected by the capacitors.  For more 
information on the changes in later model IIe computers, refer to Apple IIe 
Technical Note #9, Switch Input Changes.


Hopefully, this Note has given the reader a good feel for the paddle circuitry 
and the routines which determine the paddle values.  To reinforce the material 
covered, you should try writing your own paddle read routine.  For example, 
you could write a read routine that would read two paddles at once.  The 
software loop will not have the 11 Ásec. resolution of the PREAD routine, but 
you will find it still works just fine.  Happy programming.

Further Reference
    o    Apple IIe Technical Reference Manual

All content Copyright ©1984-2003, Terence J. Boldt, unless noted otherwise.