13.2.5 PowerOnReset

Modification to the ST power-on reset delay time

Jeff Rigby - Intersect Software - 06/24/88

The following modification will cause the ST to be in a Halt condition for apx 14 sec following turn-on. This allows a Hard Drive time to go through its initialization. The modification will not affect reset timing (.3 sec) It's relatively simple in that it requires the replacement of only one re- sistor (in the 520 & 1040 series).

If you have a Hard Drive for your ST you presently have to turn on the HD, wait until it stops making noise (initialization...about 14 sec) and then turn on your computer. With the circuit modification below you can now turn both on at the same time (idiot proof).

520ST and 1040ST

All computers have reset circuts and a circuit to perform a reset after the computer has been turned on (allowing the power supply to stabilize). In the ST these two reset circuits are in one chip, a 556 timer IC (a 556 is two 555 timers in one package). Both circuits use the same timing com - ponents for a delay of .3 seconds. One circuit holds the reset low for .3 sec after the reset button has been pushed and the other holds the reset low for .3 seconds after power turn-on. This second circuit (power on re - set) is the one we are going to change.

Basically we are looking at a 22Uf cap charging from B+ through a resistor (12k). When the voltage on the cap reaches trigger level the 555 timer turns off allowing the reset line to go high.

The formula for Time T with Cap C and Resistor R is as follows:

        Resistor = R
        Cap      = C   T= (1.1)*R*C
Time (hold down) = T

      For a stock ST
                   R = 12k
                   C = 22Uf

           Thus:   T=(1.1)*(12000)*(0.000022)
                   T=.29 sec

Ok now for your computer....

Turn on your Hard drive and count the seconds until the activity light goes out. Mine is about 14 seconds. This time is what you need to determi- ne the value of the resistor you are going to add to your ST.

For a time of 14 sec we use the following formula to determine the resis - tor we need.

      R= (14 sec)/(1.1)*(0.000022)
      R= 578k

Look in your ST for a Chip that has the number 556 on it (NEAR THE RESET BUTTON). Off pin 8 you will find a resistor with the color bands; brown, red, orange (12k).

     520ST R83
    1040ST R9

Cut this resistor loose and install a 560K resistor; green, blue, yellow.

Now reassemble and check. Extremes of temperature can affect the timing of this RC circuit. I have found that a very cold computer (40F) can knock almost two seconds off the circuits reset time. Also, the 22Uf cap is ma - nufactured with a tolerance of 20%. A 20% difference can make a 4 second difference in your calculations. You may want to install a variable resis- tor in place of a fixed resistor. If you are like me, you can't wait for your computer to come on and you will want the min wait time that still allows the HD to initialize properly. The variable resistor will allow you to set the time to the nearest second. Use a 360K resistor in series with a 500 K pot.

MEGA ST

Atari changed the design of the Reset circuit in the Mega ST. There are no longer two seperate circuits for reset. I had to kluge my Mega to get the same functionality. See below:

       --| |--*
         c3   |
              |         1000Uf               1000Uf @ 16v   ($1.50)
              *---|>|-*--| |----* Gnd
               diode  |         |            1 amp diode    (.50)
                      *--{}{}---*
                      |  33k ohm             33k ohm @ 1/4 watt  (.50)
                      |
                      *--|>|-----* 5v
                       diode                 1 amp diode    (.50)

C3 is a 10Uf cap under the switching power supply. This cap is charged by a resistor network similar to the 555 circuit in the 520ST. When the vol - tage reaches 1.2v the reset line goes high. The circuit has the same .3 sec delay for reset and 13 seconds for turn-on.

The above circuit connects to the Mega at three places: 5v, Ground and the positive terminal of C3.

Increasing the value of the 1000Uf cap increases the turn-on reset delay. The 33K resistor is used to fine tune the reset delay for power turn-on.

Increasing the value of the resistor decreases reset delay and decreasing the value increases power on reset delay. Typical resistor values are from 18k (19 sec) to 50k (11 sec).

The bottom diode discharges the 1000Uf cap to the 5v supply (when you turn off the computer the 5v supply goes to 0) when you turn the Mega off. You should count to 5 before turning a Mega back on to insure a complete cold boot. Waiting 1 sec will not allow the 1000Uf to discharge enough (the 5v supply in the Mega discharges slowly after dropping to .6v) and your power -on reset delay may drop to 9 sec from 13 sec.

The top diode provides isolation so that pressing the reset button after the 1000Uf cap has charged (power on) will cause only a .3 sec reset.

The above modifications require some technical skill and should be done by a service center or a qualified person. Intersect Software makes no gua - rantees reguarding the reliablity of the above modifications. You, as al - ways, perform the modifications at your own risk.

This technical note may be freely copied as long as the credits remain in- tact.

Jeff Rigby
Intersect Software
3951 Sawyer Rd.
Sarasota, Fl. 34233