Re: Signal generation on parallel port
From: MS. Mo (mdfmk_at_perso.be)
Date: 11/19/04
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Date: Fri, 19 Nov 2004 20:26:07 +0100
On Fri, 19 Nov 2004 22:45:25 +1100
<HLHL> wrote:
> I want to create square wave signal on parallel port with programmable
> frequency between 10-100Hz.
>
Few times ago, I had to write a software that does control an old adc, connected to an old SPP parallel port.
I did'nt had to use EPP/ECP so I dunno if it works too, but you can check that by yourself with an oscillo :)
The || port is composed of 25 pins, and part of those pins can be controled separately by
throwing bits in the parallel port registers.
in outgoing way,
1 mean +5V
0 0V
first here's the pin assignation:
Pin No (D-Type 25) SPP Signal Direction In/out Register
1 nStrobe In/Out Control
2 Data 0 OUT Data
3 Data 1 OUT Data
4 Data 2 OUT Data
5 Data 3 OUT Data
6 Data 4 OUT Data
7 Data 5 OUT Data
8 Data 6 OUT Data
9 Data 7 OUT Data
10 nACK IN Status
11 BUSY IN Status
12 PaperO/E IN Status
13 Select IN Status
14 nAutoLF IN/out Control
15 nError/nFault IN Status
16 Initialize IN/out Control
17 nSelect Printer/ nSelectin IN/out Control
18-25 ground gnd
then to access the parallel port you'll have to use a special adress called the BASE adress
usually located @ 378h.
To create a square wave, on a certain pin, all you have to do is writing a 1 at the
corresponding bit in the right register. The correspondance bit <-> pin is shown below
in BASE+0 (Data register)
you can control all the data bits, bit 0 <--> Data 0 and so on
their main purpose is to send information to a peripherial device so you might use those
to generate your wave.
in BASE+1 (status register)
Bit 7 Busy
Bit 6 Ack
Bit 5 Paper Out
bit 4 Select
bit 3 Error
bit 2 IRQ (tell the system to use IRQ or not)
bit 1 and 0 are reserved (?)
and the last
BASE+2 (Control port)
bit 7 and 6 are unused
bit 5 enable bidirectionnal mode
bit 4 enable irq via ack line
bit 3 select printer
bit 2 init printer
bit 1 auto linefeed
bit 0 strobe
so now you have all the register <-> physical pin association
The registers are used via 2 kernel functions, inb (read the register) and outb (write on it)
those are fast but require to run as root (other people use read/write on /dev/lp)
and require also to use ioperm to tell the kernel to allow the application to access the port.
here's an example:
#define BASE 0x378 /* || port base adress */
then use ioperm to grant the permissions:
int ioperm(unsigned long from, unsigned long num, int turn_on);
in your case you'll only use the base address
if(ioperm(BASE,1,1)) {perror("Ioperm b0rked"); exit (0);}
and then you can use
outb function to create your square with a loop
it might look like this:
char byte = 0;
int i;
for(i = 0; i < 1000; i++){
outb(byte,BASE);
byte ~=byte; /* alternate 0 - 1 */
usleep(10000); /* use this for the frequency adjustments */
}
the time specified for usleep is in microseconds.
You might also have a look at beyond logic
http://www.beyondlogic.org/spp/parallel.htm
> Can one of the Linux guru guide me what is the best way of generation of
> squeare wave with highest possible frequency stability regardless of the
> process load on the computer?
>
Dunno if I'm clear, but
I hope it helps :)
Mo
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