So this keeps coming up a few times a year, almost like people completely forget about how to use Google every now and then.<\/p>\n
Someone has a Williams 6809 hardware game like Defender, Stargate, Joust, etc.\u00a0 They install a switcher instead of repairing the existing linear supply (which is\u00a0 OK, people have their reasons), and everything works great, except<\/em> for the fact that they occasionally get CMOS corruption and lose things like settings, bookeeping, and scores.<\/p>\n Is something wrong with the switcher?\u00a0 No, something is just wrong with the CPU.<\/p>\n <\/p>\n The Motorola 68xx and early 68k (I believe) had a really nice feature<\/em>.\u00a0 Once running (out of reset) if the supply voltage drops below the minimum, the CPU essentially starts executing random instructions and can stomp all over memory.\u00a0 (Not enough voltage to sustain dynamic registers<\/em>, perhaps?)<\/p>\n If it tries writing ROM, no effect.\u00a0 Writing to volatile RAM?\u00a0 Game is going down anyway, so no big deal.\u00a0 Writing to your CMOS data?\u00a0 Now<\/em> you have a problem.\u00a0 The design of Williams’ hardware included a CMOS protection circuit (actually 2 kinds of protection, but we are talking about the power-related one).\u00a0 This circuit detects the voltage drop from the game turning off (or whilst powering on) and prevents writes to the CMOS so it does not get stomped by a malfunctioning CPU.<\/p>\n How does it work? By monitoring the +12v<\/strong> line.\u00a0 Due to how the original linear supply works, the +12v<\/strong> line will start to drop before the +5v<\/strong> line.\u00a0 The CMOS protection detects this and turns on the CMOS write protection before the +5v<\/strong> drops low enough to make the CPU start going crazy.<\/p>\n When you upgrade to a switcher, you change the behavior of the +12<\/strong> line – instead of dropping before the +5v<\/strong>, it now drops at the same time<\/em><\/strong>.\u00a0 With this new behavior the CMOS protection stuff cannot kick in before the +5v<\/strong> drops too low.\u00a0 The result?\u00a0 Those random CPU writes might eventually write to the CMOS memory, corrupting it.<\/p>\n The fix is easy: get the +12v<\/strong> line to drop before the +5v<\/strong> line, like on the linear supply, so the CMOS protection has time to kick in.\u00a0 Adding a cap (4700-18000uf) across the +5v<\/strong> supply should keep it up long enough so that the +12<\/strong> drops before it and the protection circuit can do its job. Et voil\u00e0<\/em> – your CMOS corruption problem is fixed.\u00a0 An old idea I had was to put a CPU supervisor in place (like the Maxim MAX* series) to handle a low-voltage situation.<\/p>\n Update: I decided that this was a problem that could use a better solution along the lines of a drop-in solution and brought the old idea back to life.\u00a0 See here<\/a> for more info.<\/p>\n","protected":false},"excerpt":{"rendered":" So this keeps coming up a few times a year, almost like people completely forget about how to use Google every now and then. Someone has a Williams 6809 hardware game like Defender, Stargate, Joust, etc.\u00a0 They install a switcher instead of repairing the existing linear supply (which is\u00a0 OK, people have their reasons), and […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3,58],"tags":[84,85,59],"class_list":["post-295","post","type-post","status-publish","format-standard","hentry","category-repair-logs","category-williams","tag-cmos","tag-switcher","tag-williams"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t