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Using setup_spi

 
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kgng97ccs



Joined: 02 Apr 2022
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Using setup_spi
PostPosted: Sun Apr 09, 2023 10:04 pm     Reply with quote

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.
PCM programmer



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PostPosted: Mon Apr 10, 2023 6:01 am     Reply with quote

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.
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Mon Apr 10, 2023 10:41 am     Reply with quote

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
kgng97ccs



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PostPosted: Tue Apr 11, 2023 1:20 am     Reply with quote

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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