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kgng97ccs
Joined: 02 Apr 2022
Posts: 97
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| Using setup_spi |
 Posted: Sun Apr 09, 2023 10:04 pm |
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We have an old program that uses the PICF45K20 MCU and has the following statement: | Code: | | setup_spi(SPI_MASTER | SPI_CLK_DIV_4 | SPI_SS_DISABLED | SPI_L_TO_H); |
Immediately after the statement, we set a breakpoint to check the SSPCON1 register setting, which shows 00100001.
This setting suggests that the SPI is running at a clock of FOSC/16 instead of the FOSC/4 set in the setup_spi() function?
Is it correct that the SPI_SS_DISABLED parameter should not be there?
We compiled the program using CCS C compiler v5.115 on MPLAB IDE v8.92.
I will appreciate any comment or advice. Thank you. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon Apr 10, 2023 6:01 am |
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SPI_SS_DISABLED is only used as a setting for an SPI Slave. You are using
the PIC as an SPI Master, so SPI_SS_DISABLED should not be there.
Remove it. |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19496
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| Posted: Mon Apr 10, 2023 10:41 am |
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and on the clock, read the data sheet.
The SPI peripheral is fed with the master oscillator/4.
The setup sets the prescaler _from this_.
So selecting a prescaler of /4, sets the SPI to run off FOSC/16 |
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kgng97ccs
Joined: 02 Apr 2022
Posts: 97
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| Posted: Tue Apr 11, 2023 1:20 am |
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Thank you, PCM Programmer and Ttelmah, for your input.
| Ttelmah wrote: | The SPI peripheral is fed with the master oscillator/4.
The setup sets the prescaler _from this_.
So selecting a prescaler of /4, sets the SPI to run off FOSC/16 |
When we examined the #define constants provided in the CCS 18F45K20.h file for the setup_spi() function, we can see that the constants make the x in SPI_CLK_DIV_x match the intended divisor.
For example, SPI_CLK_DIV_4 is defined as 0, and gives the resulting SSPM bits of the SSPCON1 register as 0000, which refers to an SPI clock of FOSC/4. Similarly, SPI_CLK_DIV_16 is defined as 1, and SPI_CLK_DIV_64 is defined as 2, and both give the intended clocks FOSC/16 and FOSC/64 respectively, as confirmed by the resulting SSPM bits.
In addition, Figure 17-1 (page 180) of the PIC18F45K20 datasheet indicates Tosc as the input to the MSSP (followed by a prescalar of 4, 16, or 64), and we understand that to mean FOSC.
I wonder if our understanding is correct. |
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