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I don't understand smart pin pull-down.
John Abshier
Posts: 1,115
in Propeller 2
I am using the Parallax Control board. I have a program that loops and reads status of the button on P20. If I set the pull-down outside the loop it doesn't work. If I call the SetPulldown method immediately before reading the button it doesn't work the first time thru the loop. If I add a delay after the call to SetPulldown it works. I guess I should be happy that I have something that works, but I don't understand what is happening.
Here is the code with results of different test cases in comments.
Here is the code with results of different test cases in comments.
CON
oscmode = $010c3f04 'standard stuff
freq = 160_000_000
baud = 230400 'must configure RUN command to match this
thepin = 20
OBJ
ser: "PrintfSerial" 'access to Propellor output (output only)
pub main | pinInput
clkset(oscmode, freq)
ser.start(baud) 'start up serial terminal
' SetPulldown(thepin) ' if here 8 to 10 lows then highs with no button push
waitcnt(freq * 2 + cnt) ' wait to enable terminal
ser.str(string("Pullup Pulldown Input test"))
ser.nl
repeat
SetPulldown(thepin) ' without delay below 2 highs then lows and highs with pin pushed
waitcnt(freq / 8_000_000 + cnt) ' works with this delay. freq / 8_000_000 has one high then lows and highs with pin pushed
pinInput := In(thepin)
ser.dec(pinInput)
ser.nl
waitcnt(freq / 10 + cnt)
PUB SetPulldown(pin)
asm
wrpin ##( %0_011_000010010 << 8 ), pin
drvl pin
endasm
PUB In(pin) : state
asm
dirl pin
testp pin WC
if_c addx state, #0
endasm
Comments
But, I think this is a smartpin mode, so you need to set dir high on this pin after the wrpin and leave it high.
Then, hopefully, you've picked the smartpin mode where IN gives you the state of the pin.
http://forums.parallax.com/discussion/comment/1464414/#Comment_1464414
Right, what is happening here ... is:
1: SetPullDown() drives low with 15kR pulldown. This takes time, and not only because it's a weak drive but also because there is a number of stages involved in both controlling the pins and reading them back in again.
2: Then DIRL stops the driving. This also takes time so the immediate TESTP still works, but any further tests will show no pulldown. ("Direction", DIR, is somewhat misnamed. It's less of a direction control and more just an output drive control, it would have been more correctly named Drive or Out-Enable.)
The time factor involved is not unique to I/O operations. There is many operations in all hardware that take longer than one instruction to come to fruition. How that is managed varies:
RDFAST instruction, for example, has two ways to manage its FIFO pre-filling requirements. One is to stall execution until the pre-filling is complete and only then is the next instruction allowed to continue. The other way is it can be told to initiate the pre-filling but allow execution to continue even though the FIFO is not yet ready to be used.
The first method doesn't require any precautions from the developer. But it costs execution time.
The second method requires the developer to understand that the FIFO still needs time to be ready before use. But it has the advantage of no-stalls in execution.
The pins can switch between weakly driving high and low just on OUT change. That's why there is HHH and LLL separately in the config bits. They can be resistances or constant currents or mixed with open collector or logic drive. If HHH and LLL are both %111==Float then DIR and OUT no longer has any effect on the output.