Timeline for Impedance of microcontroller pins with no power supplied?
Current License: CC BY-SA 3.0
10 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jan 30, 2012 at 20:20 | vote | accept | pingswept | ||
| Jan 30, 2012 at 18:51 | comment | added | pingswept | @OlinLathrop: I could decrease the series resistor to 1k, which would mean 95% rise time in 60 ns, which seems OK on my 1 MHz serial clock. But this all seems risky enough that I think I may stick with the slower JTAG programming I've got. | |
| Jan 30, 2012 at 18:44 | comment | added | markrages | @pingswept 50 uA? Maybe the micro hold weak pull-ups while in reset. Anyway, 50 uA should not be a problem for your serial flash programmer. | |
| Jan 30, 2012 at 18:26 | comment | added | Olin Lathrop | @pings: Figure 10 pF to ground at least, 20 would be safer. Then compute the time constant and the resulting rise time. For example, 10 kOhms x 20 pF = 200 ns. The 95% rise time is 3 time constants, or 600 ns in this case. You also need to consider noise pickup on a relatively high impedance digital node like that. | |
| Jan 30, 2012 at 18:08 | comment | added | pingswept | @markrages I could decrease the series resistors to 1k. Is there a way I can estimate the effects? The traces are all 5 mil wide with length around 0.5 inches, mostly about 30 mil from a ground plane. | |
| Jan 30, 2012 at 17:53 | comment | added | markrages | @pingswept, the static impedance of the pins isn't what is important, it is the capacitance of the inputs. The capacitance will combine with the 10k resistors to form a low-pass RC filter. This is bad for a high-speed data line. Like a serial flash is likely to have. | |
| Jan 30, 2012 at 17:47 | comment | added | pingswept | @markrages: On one line where I have an LED hooked up, this micro appears to leak ~50 uA both in reset and bootloading. Stops once Linux boots and the sysfs driver takes over. Kind of weird. | |
| Jan 30, 2012 at 17:43 | comment | added | pingswept | I can't easily do method 1 because the serial flash holds the program the micro will run, so method 2 is more appealing. If I put 10k resistors in series with all the serial flash data pins, that would limit the current to ~0.27 mA when I pulled them high, and I'm confident that wouldn't burn anything. I'd expect communication between the chips would still work because 10k is small compared to the ~MΩ impedance of all the inputs. Does that sound sensible? | |
| Jan 30, 2012 at 17:37 | comment | added | markrages | Further to point #1. Most microcontrollers will keep their IO lines in high-impedance state while in reset. So you may only need to ensure the micro is held in reset while programming the other part. | |
| Jan 30, 2012 at 17:25 | history | answered | Olin Lathrop | CC BY-SA 3.0 |