gnupic: Thread: gpsim stimulus

hello,

i am hoping that someone can help me. I have been experimenting with
stimulus files with gpsim 00.20.1.  I just can't get things to work as I
expect for example the following file:

processor p16f84

load -s test1.cod

node test_node

stimulus asynchronous_stimulus

start_cycle 0x10
initial_state 1
period 0x50

0x20  0
0x25  1
0x30  0

name asy1

end

attach test_node asy1 portb0


My understanding is that this would result in portb0 going high at 0x10
then low at 0x20, high at 0x25, low at 0x30, then high again at 0x60 and
repeating

What I get is high at 0x25, low at 0x30, then high again at 0x75.  It
appears that the initial_state of 1 is not occurring.  When gpsim starts
I get.

new node : test_node
period = 50
phase = 0
start_cycle = 0
Next break cycle = 20

What have I missed??

regards

John

On Thu, 10 Aug 2000, John Duncan wrote:

> hello,
> 
> i am hoping that someone can help me. I have been experimenting with
> stimulus files with gpsim 00.20.1.  I just can't get things to work as I
> expect for example the following file:
> 
> processor p16f84
> 
> load -s test1.cod
> 
> node test_node
> 
> stimulus asynchronous_stimulus
> 
> start_cycle 0x10
> initial_state 1
> period 0x50
> 
> 0x20  0
> 0x25  1
> 0x30  0
> 
> name asy1
> 
> end
> 
> attach test_node asy1 portb0
> 
> 
> My understanding is that this would result in portb0 going high at 0x10
> then low at 0x20, high at 0x25, low at 0x30, then high again at 0x60 and
> repeating
> 
> What I get is high at 0x25, low at 0x30, then high again at 0x75.  It
> appears that the initial_state of 1 is not occurring.  When gpsim starts
> I get.
> 
> new node : test_node
> period = 50
> phase = 0
> start_cycle = 0
> Next break cycle = 20
> 
> What have I missed??

The latest stuff in CVS.

The initial state thing was broken.

Your understanding is almost correct - but your misunderstanding is no fault of
yours. gpsim's stimuli are powerful, yet extremely difficult to use...

Let me clarify each of the parameters for an asynchronous stimulus.

initial_state - This is the state that the asynchronous stimulus will be in
until the stimulus is driven to a new state.

start_cycle - This is the pic instruction cycle when the asynchronous stimulus
will begin to function

phase - not used and will be removed

period - the number of cycles for repeating the stimulus

So let's take your example and see what it is supposed to do:

> start_cycle 0x10
> initial_state 1
> period 0x50
> 
> 0x20  0
> 0x25  1
> 0x30  0
> 

When the stimulus is created, it will be driven high. It will remain high until
the first sample drives it low. This occurs at start_cycle  + sample[0].cycle or
0x10 + 0x20 = 0x30. At cycle 0x10+0x25 = 0x35, it will be driven high. And
finally at cycle 0x40 it will be driven low.

The period is 0x50 cycles, so at cycle 0x60 the stimulus repeats. At cycle 0x60
+ sample[0].cycle = 0x60 + 0x20 = 0x80, the stimulus will be driven
low. However, it is already low from the previous period. Note that the
"initial_state" of 1 doesn't mean that the stimulus will go high at 0x60. At
cycle 0x85, the stimulus goes high, 0x90 it goes low.

If I wanted to create a pulse stream to drive the RX line on the UART I'd do
something like this (actually I'd write a script to do it for me...)

# send a string of 0x55's to the UART

# assume about 9600 baud - bit time ~ 100 cycles

stimulus asynchronous_stimulus

start_cycle 10000  # wait a while for the system to come up
initial_state 1    # quiescent state
period 4000        # Send a byte every 4ms or so

0     0     # start bit

100   1     # lsb
200   0
300   1
400   0
500   1
600   0
700   1
800   0     # msb

900   1     # stop bit

name uart_stim

end
 
Does that clear it up a little? 

Scott