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Hi List, We're contemplating using the 9S12D64 in a test application where using the internal pullup and pulldown devices could be very convenient if we knew the range and consistency of the resistance values of these devices. Otherwise we need a whole pile of external resistors and switches. Essentially, I'd like to use these devices to wiggle the load on a processor input pin a littel bit and measure the effect on the attached circuit. Only very limited accuracy is required. The datasheet does not seem to be much help. For example, The pullup device current is claimed to be a maximum of 130uA at Vil max. This suggests an effective resistance of at least 0.35 * 5V / 130 uA = 13K. Does this hokey analysis seem reasonable? And does anyone have any idea what the maximum value might be? Or how consistent the value might be with time and temperature? Processor-to-processor variation (or even pin-to-pin variation within the processor) does not matter much as it can be calibrated out to some extent. Any insights appreciated, Pat |
HCS12 Pull Devices
Started by ●March 18, 2005
Reply by ●March 19, 20052005-03-19
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Haven't tried this with a D series, but I am working on a E64 keyboard controller that senses button presses on Port AD which is set to digital input and pulled high internally. I noticed today that I could pull these inputs to logic lo with a 4700 ohm resistor connected to ground, but not with a 20000 ohm resistor. Sorry, didn't measure the divider voltage. Bill T. http://www.kupercontrols.com --- In , Pat Fitzpatrick <pat.fitzpatrick@a...> wrote: > Hi List, > > We're contemplating using the 9S12D64 in a test application where using > the internal pullup and pulldown devices could be very convenient if we > knew the range and consistency of the resistance values of these > devices. Otherwise we need a whole pile of external resistors and > switches. Essentially, I'd like to use these devices to wiggle the load > on a processor input pin a littel bit and measure the effect on the > attached circuit. Only very limited accuracy is required. > > The datasheet does not seem to be much help. For example, The pullup > device current is claimed to be a maximum of 130uA at Vil max. This > suggests an effective resistance of at least 0.35 * 5V / 130 uA = 13K. > > Does this hokey analysis seem reasonable? And does anyone have any idea > what the maximum value might be? Or how consistent the value might be > with time and temperature? Processor-to-processor variation (or even > pin-to-pin variation within the processor) does not matter much as it > can be calibrated out to some extent. > > Any insights appreciated, > > Pat |
Reply by ●March 19, 20052005-03-19
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Pat, I'm sure HCS12 pullup/downs aren't passive resistors. They must be somewhat made of MOS transistors and aren't linear. Also I think you used bad current or voltage values while calculating effective resistance. Ipulup @ Vilmax -130 Vilmax is 0.35Vdd. The voltage applied to the terminals of pullup "resistor" is Vdd-0.35Vdd. So the minimal effective resistance at Vilmax is: Reff=Vdd(1-0.35)/130uA=5*0.65/130u%k Ipulup @ Vihmin -10 (0.65Vdd) Reff=Vdd(1-0.65)/10uA5k Similarily voltage on pulldown is Vin-Vss Ipuldown @ Vihmin 130 (0.65Vdd) Reff=Vdd*(0.65-0)/130uA%k Ipuldown @ Vilmax 10 (0.35Vdd) Reff=Vdd*(0.35-0)/10uA5k You nay see how nonlinear pullups are. Though datasheet talks only about min/max currents and you may only guess what effective resisance will be at say at 2.5V. Regards, Edward ----- Original Message ----- From: "Pat Fitzpatrick" <> To: <> Sent: Friday, March 18, 2005 8:36 PM Subject: [68HC12] HCS12 Pull Devices > > Hi List, > > We're contemplating using the 9S12D64 in a test application where using > the internal pullup and pulldown devices could be very convenient if we > knew the range and consistency of the resistance values of these > devices. Otherwise we need a whole pile of external resistors and > switches. Essentially, I'd like to use these devices to wiggle the load > on a processor input pin a littel bit and measure the effect on the > attached circuit. Only very limited accuracy is required. > > The datasheet does not seem to be much help. For example, The pullup > device current is claimed to be a maximum of 130uA at Vil max. This > suggests an effective resistance of at least 0.35 * 5V / 130 uA = 13K. > > Does this hokey analysis seem reasonable? And does anyone have any idea > what the maximum value might be? Or how consistent the value might be > with time and temperature? Processor-to-processor variation (or even > pin-to-pin variation within the processor) does not matter much as it > can be calibrated out to some extent. > > Any insights appreciated, > > Pat > Yahoo! Groups Links > > |
