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nurquhar
Joined: 05 Aug 2006
Posts: 149
Location: Redditch, UK
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| 12F1840 Baudrate not setup with BRG16=1 ? |
 Posted: Mon Jan 06, 2020 4:31 am |
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CCS version 5.025
I have been working on an application which uses the USART on the 12F1840 at 115,200 baud.
I used these values to setup the USART:
| Code: |
#use delay(INTERNAL=32MHz)
// Default Uart TX = A0, RX = A1
// Alternate Uart TX = A4, RX = A5
#use rs232(UART1, baud=115200,parity=N,bits=8,stop=1, ERRORS)
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When I print out the relevant registers for the baud rate I see CCS has done it with these values :
SYNC=0, BRGH=1, BRG16=0, SPBRG=16
According to the 12F1840 datasheet (p281) it is working with an error rate of 2.12% and an actual rate of of 117.64k
HOWEVER
The datasheet says on page 283 that with BRGH=1 and BRG16=1 and an SPBRG value of 68 you can get an error rate of just 0.68% !!
So why does the compiler not use this better option for the 12F1840 ?
As additional information I got here by wanting to reduce the clock rate to 8Mhz instead of 32MHz but CCS refused to compile even though as stated on page 283 that 115,200 can be achieved with an error 2.12% if you use BRGH=1 and BRG16=1.
If I change the device to the similar part 16F1825 then you can get CCS to compile for 115,200 baud @ 8Mhz. So does CCS not know the 12F1840 has the BRG16 bit ?
If I force my own setting of BRGH=1, BRG16=1 and SPBRG=68 then the USART works fine at 115,200, as it also does with the CCS setup but mine is more accurate I assume. Also I can reduce my clock to 8Mhz if I force my own baud rate 115,200 USART setup. Where as CCS says it can't be done.
I am using an old version of CCS Compiler 5.025 so perhaps this has been fixed since. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon Jan 06, 2020 4:41 am |
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Here is the UART init code for vs. 5.091:
| Code: | .... void main(void)
000A: MOVLW F0
000B: MOVLB 01
000C: MOVWF OSCCON
000D: MOVLB 00
000E: CLRF rs232_errors
000F: MOVLB 03
0010: BSF BAUDCON.BRG16 // Set BRG16 = 1
0011: MOVLW 44 // 68 decimal
0012: MOVWF SP1BRGL
0013: MOVLW 00
0014: MOVWF SP1BRGH
0015: MOVLW A6
0016: MOVWF TXSTA // Set BRGH = 1
0017: MOVLW 90
0018: MOVWF RCSTA
0019: CLRF ANSELA
001A: MOVLB 02
001B: CLRF CM1CON1
001C: CLRF CM1CON0
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nurquhar
Joined: 05 Aug 2006
Posts: 149
Location: Redditch, UK
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| Posted: Mon Jan 06, 2020 4:47 am |
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| Yes thanks for the quick test to confirm the CCS bug is now fixed. Looks like I should throw some money in the direction of CCS. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19509
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| Posted: Mon Jan 06, 2020 6:22 am |
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It was fixed quite a long time ago. It was just that the algorithm they used
was written originally for chips that didn't offer BRG16 as an option.
When these started to become common, they changed this, but for
quite a while this option was not used. |
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