Electronics > Microcontrollers

Need help dumping the ROM in a failing 80C49 (MCS-48)

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abyrvalg:
Do I understand correctly? - the idle Vcc sags from 5V to 1.5V? Looks like there is some pin driven low connected directly to Vcc. Which pins of 8049 are connected to Vcc now? Try disconnecting them one by one while observing the Vcc level to find which one causes the sag.

Andy Watson says “PROG must float” and you say “PROG is HI” - is it floating and driven HI by the 8049 or you drive it HI? That can be the first candidate for disconnect.

trondl:

--- Quote from: abyrvalg on March 17, 2022, 11:17:53 am ---Do I understand correctly? - the idle Vcc sags from 5V to 1.5V? Looks like there is some pin driven low connected directly to Vcc. Which pins of 8049 are connected to Vcc now? Try disconnecting them one by one while observing the Vcc level to find which one causes the sag.

--- End quote ---

According to the schematics: pin 39 T1, pin 5 /SS and pin 6 /INT / /CS are tied to VCC.
I'll try lifting them later today

Andy Watson:

--- Quote from: trondl on March 17, 2022, 10:54:28 am ---
--- Quote from: Andy Watson on February 05, 2022, 03:08:18 pm ---What are you doing with the PROG pin? Looking at an old datasheet for the Intel 8048/9 series there is a curious note under the programing and verify timing waveforms for the 8048 (I can't see an equivalent for th 8049 - so YMMV), it says:
"Prog must float if EA is low, or if T0=5V for the 8748. For the 8048 PROG must always float."

--- End quote ---
PROG is HI during the entire dump

--- End quote ---
Assuming I am looking at the correct programmer circuit, the PROG pin appears to be pulled HI (via a diode) with a 220\$\Omega\$ resistor - that's quite an aggressive pull-up, especially for a cmos device. I would not class it as "floating".

PCB.Wiz:

--- Quote from: trondl on February 05, 2022, 01:12:51 pm ---
When idle, 8749 gets a clean 5V at the i/o bus and 0V at VCC.
8049 only get about 1.5V at the bus and VCC / VDD is about 1V.
...

Bought an almost cheap second unit from japan in the hope that dumping the second MCU would be a breeze.
Not so.
The dumping waveforms are almost exactly like the failing one.
It just seems like the LO bits never reach ~0V and VCC is also idle at 1.5V, just as the Data pins are doing.


--- End quote ---

Something is very awry, if Vcc droops to 1.5V - Check the current demand ? and compare your 8749 with 8049's
Those old parts are NMOS so they need a lot of supply current

You might also want to check the pin-mapping, to make sure it really is an 8049 ?

trondl:

--- Quote from: PCB.Wiz on March 20, 2022, 04:17:40 am ---
Something is very awry, if Vcc droops to 1.5V - Check the current demand ? and compare your 8749 with 8049's
Those old parts are NMOS so they need a lot of supply current

You might also want to check the pin-mapping, to make sure it really is an 8049 ?

--- End quote ---

Thanks a bunch again for all the help from everyone so far!
Things are busy as usual, resulting in slow progress.

Yes, it is definitely a 8049 I'm dumping.
I've attached screenshots from the SDE1000 service manual and the Arduino shield for comparison.
When desoldering the "new" second chip, pin 34 (P1-7) was bent inwards / floating.
No Idea why that specific pin was in that state compared to the other unit I have, where all pins where soldered.

I need to clarify one thing here.
When the 8749 is idle, the only pins that are held high/5V are the DB0-7 pins.
VCC is at 0V then.
The 8049 is when the Ardino is fully restarted at 0V on all pins (even on the VCC and DB pins, but on successive read it seems like it enters a semi-stuck state with 0.9 to 1.5V on various pins (listed bellow).
I repeat that VCC should be 0V, not ~1V when idle.

I've also tried to lift some pins with no direct results: 25 Prog, 5 SS, 6 INT, 39 T1, 11 ALE.
Comparing which pins are absolutely necessary with this other project:
https://www.sbprojects.net/projects/8049spy/index.php

While at it, I made a list of which pins are stuck after a dump.
A curious discovery is at pins 27 & 28 (P1-1 / 2) is tied to ground, while the rest of P1 up to pin 34 is floating and has the stuck voltage.
Would it be safe to tie the rest of the P1 pins to ground?
Could there be more pins that could be tied low instead of floating?
Could VDD be disconnected?

Description:
Pin#, _x_ after pin indicates >0.9V present, Name, Notes in reference to Arduino shield schematics.

1   T0
2   xtal
3   xtal
4   Reset
5x   SS
6x   INT
7x   EA
8x   RD    floating
9   PSEN
10x   WR   floating
11   ALE
12x   DB0
13x   ..
14x   ..
15x   ..
16x   ..
17x   ..
18x   ..
19x   DB7
20   Vss

21   P2-0
22   ..
23   ..
24   P2-3
25x    Prog
26x   Vdd
27   P1-0 Gnd tied
28   ..   Gnd tied
29x   ..   floating
30x   ..   floating
31x   ..   floating
32x   ..   floating
33x   ..   floating
34x   ..   floating
35x   P2-4 floating
36x   ..   floating
37x   ..   floating
38x   P2-7 floating
39x   T1
40x  Vcc

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