Jasmin hardware bug
Posted: Fri Apr 01, 2016 2:22 am
I was looking at the Jasmin schematics to prepare the Cumulus Rev2 for supporting the Jasmin and I've noticed a really strange thing on that device, I don't understand why they've done that, but basically, on both Jasmin 1 and Jasmin 2 (they are in fact the same schematics) they have a gate that drive the 6502 A2 line...
If you look at the two know schematics here : http://www.teaser.fr/~amajorel/jasmin/
The first for the Jasmin 1: http://www.teaser.fr/~amajorel/jasmin/j ... 100dpi.png
The Jasmin2: http://www.teaser.fr/~amajorel/jasmin/j ... 100dpi.png part of U9 where the output connected to U14 input and... A2
What does that mean? Simple that that NOR gate will force A2 to be high or low, A2 is a mono directional bus, from the 6502 to the rest of the world, the 6502 always drive this line, so basically what that does is that it create a contention on the A2 line when the NAND gate try to pull A2 high and the 6502 low, or when the NAND try to pull low and the 6502 high.
It could have a lot of nasty effect like at worst killing the A2 output of the 6502, or the one on the NAND gate, depends on who pull the line low.
I don't have the hardware by hand to check if this part is true or not, and if they added a patch to prevent that, but that a really bad design.
Can someone that have a Jasmin 1 or 2 near him could check that the schematic and the hardware match?
If you look at the two know schematics here : http://www.teaser.fr/~amajorel/jasmin/
The first for the Jasmin 1: http://www.teaser.fr/~amajorel/jasmin/j ... 100dpi.png
The Jasmin2: http://www.teaser.fr/~amajorel/jasmin/j ... 100dpi.png part of U9 where the output connected to U14 input and... A2
What does that mean? Simple that that NOR gate will force A2 to be high or low, A2 is a mono directional bus, from the 6502 to the rest of the world, the 6502 always drive this line, so basically what that does is that it create a contention on the A2 line when the NAND gate try to pull A2 high and the 6502 low, or when the NAND try to pull low and the 6502 high.
It could have a lot of nasty effect like at worst killing the A2 output of the 6502, or the one on the NAND gate, depends on who pull the line low.
I don't have the hardware by hand to check if this part is true or not, and if they added a patch to prevent that, but that a really bad design.
Can someone that have a Jasmin 1 or 2 near him could check that the schematic and the hardware match?