> Excellent fun! It sees if the light pen interrupt can be triggered > between raster lines x'136 and 2; if so, it's a 6569R1, if not, not. Looking at the die pictures (6569R1 and 8565R2), the only difference I see is that the 6569R1 triggers ILP on the rising edge of the (cooked) LP input, while the newer chip is level triggered. The cooking is the same for both: it outputs a single ~250ns pulse when the LP pin is asserted for the first time in a frame (except the first 1us cycle I think?) Anyway, the way that extra edge detect on the 6569R1 is implemented is buggy: it can only trigger during the second half of any 1us cycle. The test does: Clear ILP, wait for raster line 0x136; Set the LP line (via the CIAs); If that set ILP, it's not a 6569R1; Wait for raster line 2; If ILP is set now, it is a 6569R1. So it seems that the pulse you get from triggering the LP input (via the CIAs) is always missed on the 6569R1, and the pulse from having it enabled when a new frame starts is always recognised. Or something :-) Either way, the 6569R1 light pen circuit would fail to trigger the interrupt correctly; the test is detecting this bug. Segher Message was sent through the cbm-hackers mailing listReceived on 2011-12-09 21:00:08
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