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RLScott
Joined: 10 Jul 2007
Posts: 465
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| Accessing Big Endian registers |
 Posted: Sat Aug 17, 2013 2:25 pm |
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I just found out that some of the 16-bit registers in the PIC18F2331/2431/4331/4431 are Big Endian, specifically the power control PWM registers, PTPER and PDC0...PDC3. I have no idea why Microchip chose to violate their standard of Little Endian 16-bit registers, but now that they did, it seems I cannot use the 16-bit syntax in CCS C where the register is declared with a #word directive. Now I have to use the make8() macro to access one byte at a time. Am I missing something here?
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Robert Scott
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temtronic
Joined: 01 Jul 2010
Posts: 9170
Location: Greensville,Ontario
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| Posted: Sat Aug 17, 2013 5:00 pm |
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Sadly 'standards' are made to be broken ,errr, 'updated'.
Rememeber 'legacy' devices like REAL comports, 25 pin printer ports....
'We'll NEVER lose them !!'
Sigh, it's just a sign of the dynamic times we're in.
I've yet to figure out why PIC ports don't defallt to digital I/O instead of any of the peripherals that are 'muxed' to them.
Just another of life's challeneges..after all wouldn't want to get 'set in your ways' would you ???
smile..it' gets worse the older you get ...
If tubes weren't so pricey, I'd go back to them.
Jay |
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RLScott
Joined: 10 Jul 2007
Posts: 465
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| Posted: Sat Aug 17, 2013 6:53 pm |
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| temtronic wrote: |
I've yet to figure out why PIC ports don't defallt to digital I/O instead of any of the peripherals that are 'muxed' to them.
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Imagine a circuit that puts a mid-level voltage (2.5v) on a pin as soon as the PIC is powered up, because it is going to be configured as analog. If the PIC powered up assuming a digital input for this pin before it was configured as analog, the intermediate voltage could cause the PIC to draw a lot more current than it would if it were an analog pin. If the power supply is severely current limited it could even cause the device to drop below the brownout threshold and never come out of reset.
| Quote: |
smile..it' gets worse the older you get ... |
I'm 65 now. You mean it gets even worse?
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Robert Scott
Real-Time Specialties
Embedded Systems Consulting |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Sat Aug 17, 2013 7:15 pm |
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How about a little macro like this, to swap the bytes ?
| Code: |
#include<18F4431.h>
#fuses INTRC_IO, NOWDT, NOPROTECT, NOBROWNOUT, PUT, NOLVP
#use delay(clock=4M)
#use rs232(baud=9600, UART1, ERRORS)
#word PTPER = getenv("SFR:PTPERH")
#define swap_bytes(value) make16(value, value >> 8)
//=======================================
void main()
{
PTPER = swap_bytes(0x1234);
while(1);
} |
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RLScott
Joined: 10 Jul 2007
Posts: 465
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| Posted: Sat Aug 17, 2013 8:18 pm |
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I like your swap_bytes macro. It generates exactly the same code as my explicit byte moves - very efficient!
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Robert Scott
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Ttelmah
Joined: 11 Mar 2010
Posts: 19359
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| Posted: Mon Aug 19, 2013 1:58 am |
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As a comment, CCS already provide functions to directly access these registers, with the bytes correctly ordered. set_power_pwmX_duty for example, sets the PDC registers directly:
| Code: |
.................... set_power_pwm2_duty(0x1234);
0024: BSF F6E.1
0026: MOVLW 34
0028: MOVWF F77
002A: MOVLW 12
002C: MOVWF F76
002E: BCF F6E.1
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The only thing you have to 'remember' is that CCS use the logical output numbers, not the register numbers.
Best Wishes |
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RLScott
Joined: 10 Jul 2007
Posts: 465
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| Posted: Mon Aug 19, 2013 8:24 am |
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| Ttelmah wrote: | As a comment, CCS already provide functions to directly access these registers, with the bytes correctly ordered. set_power_pwmX_duty for example, sets the PDC registers directly:
| Code: |
.................... set_power_pwm2_duty(0x1234);
0024: BSF F6E.1
0026: MOVLW 34
0028: MOVWF F77
002A: MOVLW 12
002C: MOVWF F76
002E: BCF F6E.1
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The only thing you have to 'remember' is that CCS use the logical output numbers, not the register numbers.
Best Wishes |
I am really glad you posted that - not so much for the use of set_power_pwm2_duty as for the use of PWMCON1 bit 1. I hadn't realized the use of this bit in setting Duty Cycle. I did worry about the possibility that the working PDCx value would get updated inbetween my setting of the upper and lower bytes of PDCx, and thus use a totally wrong value of PDCx for one period. But after looking at the disassembly you posted I realized that this is exactly that PWMCON1 bit 1 was for.
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Robert Scott
Real-Time Specialties
Embedded Systems Consulting |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19359
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| Posted: Mon Aug 19, 2013 8:28 am |
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Yes, that is one of those 'once you see it, makes perfect sense' things. 
Best Wishes |
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