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setup_timer_0 and RTCC_8_BIT really required to work right?

 
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evsource



Joined: 21 Nov 2006
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setup_timer_0 and RTCC_8_BIT really required to work right?
PostPosted: Fri Mar 27, 2009 5:34 pm     Reply with quote
I'm really sorry if this has been covered in other topics - I did search around briefly.

I'm on a 18F2620 w/ compiler version 4.030.

Is it really necessary to have RTCC_8_BIT in setup_timer_0()? I thought since the RTCC was only 8-bit on the 18F2620 (at least RTCC_16_BIT isn't in the chips .h file), that it would just default to that.

I was getting unexpected timings, and figured out it was off by a factor of about 250-ish. I tried inserting RTCC_8_BIT in setup_counter_0, and it works fine.

Doesn't work:

Code:
setup_timer_0(RTCC_INTERNAL | RTCC_DIV_4);


Does work:

Code:
setup_timer_0(RTCC_INTERNAL | RTCC_8_BIT | RTCC_DIV_4);


Any comments on this to understand it better would be appreciated.
PCM programmer



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PostPosted: Fri Mar 27, 2009 5:44 pm     Reply with quote
Quote:
I thought since the RTCC was only 8-bit on the 18F2620

The 18F2620 data sheet says this:
Quote:
11.0 TIMER0 MODULE
• Software selectable operation as a timer or
counter in both 8-bit or 16-bit modes

REGISTER 11-1: T0CON: TIMER0 CONTROL REGISTER
bit 6 -- T08BIT: Timer0 8-Bit/16-Bit Control bit
1 = Timer0 is configured as an 8-bit timer/counter
0 = Timer0 is configured as a 16-bit timer/counter

http://ww1.microchip.com/downloads/en/DeviceDoc/39626e.pdf
evsource



Joined: 21 Nov 2006
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PostPosted: Fri Mar 27, 2009 9:39 pm     Reply with quote
[quote="PCM programmer"]
The 18F2620 data sheet says this:
Quote:
11.0 TIMER0 MODULE
• Software selectable operation as a timer or
counter in both 8-bit or 16-bit modes


Okay, lesson to be learned here: always check the datasheet. 18F2620.h (in 4.030 at least) doesn't have an option to set it up as 16 bit. It would be nice if one could assume that meant it didn't have that option.
PCM programmer



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Posts: 21708

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PostPosted: Fri Mar 27, 2009 10:03 pm     Reply with quote
Here are the constants for vs. 4.089. It's setup as a 16-bit timer if
you don't specify RTCC_8_BIT.
Code:

#define RTCC_INTERNAL   0
#define RTCC_EXT_L_TO_H 32
#define RTCC_EXT_H_TO_L 48

#define RTCC_DIV_1      8
#define RTCC_DIV_2      0
#define RTCC_DIV_4      1
#define RTCC_DIV_8      2
#define RTCC_DIV_16     3
#define RTCC_DIV_32     4
#define RTCC_DIV_64     5
#define RTCC_DIV_128    6
#define RTCC_DIV_256    7

#define RTCC_OFF        0x80 

#define RTCC_8_BIT      0x40 


The function only does one thing. It writes to the T0CON register:
Quote:
.... setup_timer_0(RTCC_INTERNAL | RTCC_DIV_1);
00028: MOVLW 88
0002A: MOVWF T0CON
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