microcontroller - Why is the Voltage only 1.4 for reset input when in high impedance? - Electrical Engineering Stack Exchange


i attempted make myself 2-in-1 programmer can program both at89cx051 , at89s52 on same board without having plugging/unplugging of port.

circuit

the main issue strobe pin aka reset (rst in circuit). (pin 1 of db25 connector). tested pc , got no results.

what seems work if connected rst vcc via 1k resistor. works because though output @ 3.8v instead of 5 (which vcc at), puts at89s52 in reset mode expected, i'm not getting 12v @ pin 1 of small at89cx051 socket.

if don't make manual connection, 1.4v measured @ rst, after added 10k resistor between vcc , rst. without rst being forced ground, should 5v i'm not.

i have tested transistors manually leg leg using diode setting on multimeter , turned out fine. resistors going base of transistors 2.2k , used 1k resistors in place of diodes. other resistors 10k. resistor going buffer (74hc125) 0 ohm.

all capacitors 0.1uf.

why getting such awkward voltages reset pin? using weak of resistors or something?

update

so ended making myself ms-dos 6.22 bootdisk , booted machine qbasic test parallel port (with port @ address 278-27ah ) in general simple circuitry , seems work decently.

when teted parallel port on pc voltmeter anode lead connected strobe pin, following results:

qbasic code           result out &h27a, 0          4v output strobe out &h27a, 1          0v output strobe

when plugged circuit port, voltage dropped:

qbasic code           result out &h27a, 0          1.56v output strobe out &h27a, 1          0v output strobe

how possible i'm getting 1.56v pin?

update 2

i disconnected 12v line circuit , voltages on reset line 1.52v , 0v

  1. you missing 10 k pull-up resistor in series 1n4148 diode @ reset pin of mpu itself. show creset being there programming ic, no 3.3 volt pull must there. reset transistor needs current on collector @ times or ic may stay in reset state.

  2. the diode prevent 12 volts going vcc 3.3 volt line during programing.

  3. pin 1 of db25 connector should have 10 k resistor ground reset transistor not turn on due leakage currents.

  4. to reset ic need ground reset pin or connect pin1 of db25 connector vcc. @ least should goal.

  5. i not sure or why 1.4 volts on reset pin. first of not defining which reset pin. ic or db25 pin 1. plus missing important pull-up , pull-down resistors standards 1.4 volts leakage current.

  6. i sure see 1.4 volt reading change , more sensible once install resistors , diode mentioned.

  7. you missed couple of important steps should working correctly, @ least reset function anyways.

  8. in honesty diagram bit confusing. looking @ left schematic db25 connector, have pin1 reset if high. cannot open collector unless pnp transistor because t1 must have base pulled low or reset mpu.

  9. you sooo close having right. yes diode forward biased 3.3 or 5 volt line, through 10 k resistor. insures mpu reset pin held high, yet can still act programming pin. 3 pin connector should not used unless programming mpu.


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