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September 28, 2015

I’ve been struggling to get the Z80 Membership card CPU board working with my prototype serial/power board. Externally, the symptom is that there’s no response to serial input.  The things I know(in no rational order) are:

  • The board works fine with the front panel and my serial adapter although it usually seems to get reset several times on startup (i.e. it displays COLD 04 or so).  I’ve also noticed a lot of bouncing on the reset line.
  • If I disconnect my adapter’s capacitor between RTS and lee’s CLR, I get COLD 00 on startup, and, with the adapter cabled to my desktop the reset pulse is clean.
  • The inverted adapter is COM12 on the desktop.
  • I sometimes get the serial terminal program on this laptop hanging trying to send to the MC.  This usually requires rebooting to free the serial port
  • The M1 signal after reset is somewhat irregular (see below).  Mostly the /M1’s are .5us but the first one is .25us.  The reset rising edge is 81ms off the screen to the left.15-09-27 m1 after reset
  • without the front panel and with only a pullup resistor connected to /reset I get a choppy reset signal on power up15-09-27 M1 post reset no FP
  • removing the pullup doesn’t clean up the signal 15-09-27 M1 post reset no FP, no pullup
  • running just power and ground to the cpu card still shows a sloppy reset 15-09-27 M1 post reset cpu board only

The bouncy period when /reset comes high is in the range of 8-15us which is VERY fast. switches have individual bounces of 10-100us.
Possibly spurious conclusion: it takes both the CPU card and FP board to generate a clean power-on reset. I would like a clean reset so i can trace instruction execution without worrying about glitches.

The circuitry attached to the /reset cpu pin on the main cpu board is meant to ensure a power on reset.
15-09-27 reset circuit

on the front panel the circuit is meant to allow the /reset to be pulled low from the CLR connector pin.
15-09-27 clr circuit

and another connection on the FP board somehow mediates the hardware reset caused by pushing F and 0 at the same time.

15-09-28 more reset

the key to understanding these is that the 74ls01 is open collector so it can’t pull UP the /RESET pin during power on but it can pull it down later.  I’ve also learned the the F 0 keys together actually cause a hardware rest – i didn’t know that!

From → Olduino/Z

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