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David M Posted
24-Mar-2003 01:17 GMT Toolset
None |  Timer David M Hi
This program i am using has two different timer routines and i dont what each one of them is doing.In the first one i do not understand what is meant by the Timer Register for the high byte(TH0) and v being shifted by 8. Also for routine number 2, what is meant by SCON = 0x50; and TMOD |= 0x20 ;.Where did the value 213 for TH1 come from(TH1=213)?
1)void timer0 (void) interrupt 1 using 1 { /* Int Vector at 000BH, Reg Bank 1 */
unsigned char i;
unsigned int v;
TR0 = 0; // stop timer
v = TL0 | (TH0>>8);
v += (unsigned int) (T0_CLOCK);
TL0 = (unsigned char) v;
TH0 = (unsigned char) (v >> 8);
TR0 = 1; // start timer
2)void main (void) {
bit CanMessageReq = 0; // Remote Frame Requested
// init serial interface
SCON = 0x50; /* SCON: mode 1, 8-bit UART, enable rcvr */
TMOD |= 0x20; /* TMOD: timer 1, mode 2, 8-bit reload */
TH1 = 213; /* TH1: reload value for 1200 baud @ 20MHz */
TR1 = 1; /* TR1: timer 1 run */
TI = 1; /* TI: set TI to send first char of UART */
// ToDo take timer from RTX51 Tiny
#define PERIOD -500 /* 250 usec interrupt period */
/* setup the timer 0 interrupt */
TL0 = (unsigned char) (T0_CLOCK);
TH0 = (unsigned char) (T0_CLOCK >> 8);
TMOD = TMOD | 0x01; /* select mode 1 */
TR0 = 1; /* start timer 0 */
ET0 = 1; /* enable timer 0 interrupt */
| | Read-Only Author
Stefan Duncanson Posted
24-Mar-2003 10:09 GMT Toolset
None | RE: Timer Stefan Duncanson v is a 16 bit integer, TH0 is an 8 bit register containing the high 8 bits of a 16 bit timer. So, to get the high 8 bits of v into TH0 you have to rightshift by 8, likewise to transfer the contents of TH0 into the high half of v you have to rightshift it by 8.
Regarding the contents of TMOD and SCON look in the device datasheet for a full explanation. The datasheet will also show you how the reload value of TH1 is calculated for baudrate generation.
Stefan | | Read-Only Author
Stefan Duncanson Posted
24-Mar-2003 10:16 GMT Toolset
None | RE: Timer Stefan Duncanson Oops! Some of that was rubbish. To transfer TH0 into the high half of v you would have to leftshift it.
I think your code was doing something different, unfortunately I can't see it while typing this reply.
Stefan | | Read-Only Author
Stefan Duncanson Posted
24-Mar-2003 10:19 GMT Toolset
None | RE: Timer Stefan Duncanson The line:
v = TL0 | (TH0>>8);
is a bit strange as TH0>>8 would always be zero. Maybe it should be TH0<<8?
Stefan | | Read-Only Author
Martin M Posted
24-Mar-2003 18:33 GMT Toolset
None | RE: Timer Martin M hi
Thanx for your help Stefan.I do not know if it is right shifting or left shifting but i will find out.One more thing, do you know what is the code below doing?
/******************************************************************************/
/* Timer 0 interrupt service function */
/* executes each 1ms @ 20 MHz Crystal Clock */
/******************************************************************************/
#define INT_CLOCK 0.001 // timer interval 0.001Sec
#define XTAL 20000000
#define X2 0 // set to 1 if in X2 Mode
#define _CLOCKVAL ((unsigned int) ((XTAL/(12/(X2+1))) * INT_CLOCK))
#define T0_CLOCK ((-_CLOCKVAL)+13) | | Read-Only Author
Martin M Posted
24-Mar-2003 18:35 GMT Toolset
None | RE: Timer Martin M hi
Thanx for your help Stefan.I do not know if it is right shifting or left shifting but i will find out.One more thing, do you know what is the code below doing?
/******************************************************************************/
/* Timer 0 interrupt service function */
/* executes each 1ms @ 20 MHz Crystal Clock */
/******************************************************************************/
#define INT_CLOCK 0.001 // timer interval 0.001Sec
#define XTAL 20000000
#define X2 0 // set to 1 if in X2 Mode
#define _CLOCKVAL ((unsigned int) ((XTAL/(12/(X2+1))) * INT_CLOCK))
#define T0_CLOCK ((-_CLOCKVAL)+13)
| | Read-Only Author
Stefan Duncanson Posted
25-Mar-2003 09:57 GMT Toolset
None | RE: Timer Stefan Duncanson Well, I'd guess it's calculating the reload value for timer 0 required to generate an interrupt approximately every millisecond with a 20MHz clock.
I think you need to have a look at a good book on 'C'. I'd recommend:
"C: A Reference Manual"
by Harbison and Steele
ISBN 0-13-326224-3
I think you should also read the datasheet for the device in conjunction with the "Intel MCS 51 Microcontroller Family User's Manual" which you can find on the Intel website.
Stefan | |
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