Couple of pull-up resistor questions/verifications
Couple of pull-up resistor questions/verifications
(OP)
1) A current PIC design uses 10k pull-up resistors and 10nF caps for bounce smoothing on a keypad. Current is obviously 500uA with the resistors, but I would like everyone's opinion on using the internal pull-ups on the PIC. Min/typical/max ratings are 50/250/400uA for each pin. Does everyone think I'm safe removing the external resistors and enabling the internal pull-ups?
2) To verify...TTL inputs tend to float high, but is there any overwhelming reason to tie them either way, and what is it? If so, is a resistor required in either case, or is a direct connection to Vcc/Gnd OK?
3) To verify...CMOS input float in no particular direction, which is bad. Again, I assume there's no difference between tieing high or low, and no resistor is required for either. Ground usually seems the logical choice since it's usually found everywhere on a typical board.
Sound about right to everyone?
2) To verify...TTL inputs tend to float high, but is there any overwhelming reason to tie them either way, and what is it? If so, is a resistor required in either case, or is a direct connection to Vcc/Gnd OK?
3) To verify...CMOS input float in no particular direction, which is bad. Again, I assume there's no difference between tieing high or low, and no resistor is required for either. Ground usually seems the logical choice since it's usually found everywhere on a typical board.
Sound about right to everyone?





RE: Couple of pull-up resistor questions/verifications
RE: Couple of pull-up resistor questions/verifications
Does anybody still use TTL? There are inputs which are TTL compatible CMOS, but these still need to be tied one way or the other.
Unused CMOS inputs can be tied without using resistors. One reason to use resistors is for ATE testing, so standard macros can be used to test each input.
RE: Couple of pull-up resistor questions/verifications
Analog debounce is the way to go if you cannot spare many cycles for software debouncing. I've used both methods with great success, but sometimes you just have to work with what you are given.
RE: Couple of pull-up resistor questions/verifications
TTFN
RE: Couple of pull-up resistor questions/verifications
TTLs were bipolar and had internal pullups by design, to get them working. There were variants between families but I've never seen a TTL going nuts because of a floating input.
I suggest that you keep you pullups and caps, and even add series resistors, not for debouncing, but to reduce EMI and ESD unless your keypad is metallized or well insulated.
RE: Couple of pull-up resistor questions/verifications
As a side question, is there any way to determine if a chip is using true TTL technology, short of seeing the detailed schematic, or should I just assume most major processors/chips made in the past 5 or so years uses strictly CMOS? Microchip leads you to believe both processes exist on their chips, but there's no way I've been able to find to confirm/deny that, other than to realize CMOS is used where power is a concern and low switching speeds are in use.
RE: Couple of pull-up resistor questions/verifications
While you can get away with floating TTL inputs, it has never been recommended practice, since enough charge on the input pin can cause a momentary noise glitch to occur.
TTFN
RE: Couple of pull-up resistor questions/verifications
RE: Couple of pull-up resistor questions/verifications
IRstuff, please correct me if I'm wrong. The transistor at a TTL input is used as two back-to-back diodes, with no amplification function of any sort, thus the internal pullup resistor can be considered the same type of load as if it was an external pullup. More, if you look at an LSTTL internal schematic, there is no more transistor, only a diode.
The susceptibility to a glitch won't be different from one of a CMOS input with the equivalent external pullup. (if the transfer function is equivalent)
RE: Couple of pull-up resistor questions/verifications
TTFN