10/28/2022 0 Comments Macspice update![]() So… where to begin troubleshooting? Based on suggestions from helpful commenters on a previous post, I used a scope to measure the Yellowstone card’s GND and 3.3V relative to a ground point on the Apple IIe motherboard. There are also 10 uF ceramic capacitors on the input and output of the LDO. The FPGA has twelve 0.1 uF ceramic bypass capacitors for each of its power/ground pin pairs, and its 3.3V supply is isolated from the rest of the board’s 3.3V supply with a ferrite bead, as recommended in the FPGA datasheet. Each 74LVC245 has a 0.1 uF ceramic bypass capacitor about 2 mm from its VCC pin. Three of the chips are configured for unidirectional signals like the address bus and control signals, and the fourth is bidirectional for the 8-bit data bus. A set of four 74LVC245 chips provide 5V to 3.3V level translation. The FPGA runs at 3.3V, powered from a Micrel MIC5504-3.3YM5-TR LDO regulator. Some hardware background: shown above is a simplified schematic of the card (click for a hi-res version). From this I concluded that the crash is happening during execution of the Yellowstone ROM code, rather than the Yellowstone card somehow causing an error with another card or with the Apple II itself. I measured times of 0.19ms, 1.57ms, and 28.5ms. During a boot-up where the computer crashes, this ROM access lasts a much shorter random-seeming amount of time. #Macspice update codeDuring a normal boot, there’s a 93ms period of near-continuous Yellowstone ROM access, which is probably running the ROM code to look for an attached drive. With a logic analyzer, I examined the pattern of card accesses during good and bad power-up sequences. #Macspice update driverMore cards means more capacitance on the data bus: maybe the Yellowstone output driver wasn’t able to switch the bus signals fast enough? More cards also means more load on the data bus: maybe the Yellowstone output driver wasn’t able to source or sink enough current to maintain a valid logic high or low voltage?
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