Conference 7.286::atarist

1083.0. "Blitter-Databus-Problems" by VNABRW::KRONSTEINER () Mon Mar 04 1991 08:44

    Hi,all Atari experts out there !
    
    Last few weeks my megST4 (tos1.4 in 6 cmos-rom's- 100ns +Blitter) showed
    some corrupted screens.
    
    The problem was seen only on my simple Basic-Program that outputs 
    Textlines with different attributes (bolt,undelined,kursiv.....) on the
    screen.
    I used a loop to show all the possible combinations with the biggest
    lettersize possible.
    On an intermittend base I got some corrupted Letters (vertical blank
    lines, sometimes in the whole column)
    Same problem in GFA and OMICRON Basic.
    Heating or cooling the CPU or spcially the BLITTER made the Problem
    solid.
    All was OK. when I switched off the blitter.
    First I assumed a problem on the motherboard but the scope on the data-
    bus showed that the problem was forced by heavy busrequest's from the
    blitter (eg. when printing kursiv text).
    Second I found out that when the databus is in the high impedance state
    it takes a lot of time for the voltage to rise. (databus has 10K
    pullups and the capacitance seems to high, maybe 90pf).
    So I changed the 74ls373 busdriver to other ones (other manufact. and
    later to 74hc373).
    This reduced the error-rate drastically next I clamped the databuslines
    at the 74ls373 with 1k to ground.(I know its a crowbar-methode)
    Since that time I am not able to reproduce the problem any more.
    
    As I mentioned before I never saw any problem when running other SW.
    
    So has anyone experienced the same problems?
    Any ideas are welcome
                          regards 
                                   Armin