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bkohan
2 Posts |
 Posted - 01 Oct 2007 : 16:22:12
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Code snippet:
SSP_CONFIG_TYPE sspConfig;
sspConfig.sspFrameSize = 8; // 8 bit frame size
sspConfig.sspClkPreDivider = 2; // SSP clock speed = 7372800 / (prediv * (1 + div))
sspConfig.sspClkDivider = 3; // = 0.9216 Mhz
sspConfig.sspClkPhase = LEADING;
sspConfig.sspClkPolarity = NEGATIVE;
sspConfig.sspFormat = SPI;
sspConfig.sspMode = MASTER;
What does "LEADING" and "NEGATIVE" mean in SPI terms? Usually Clock Phase (0/1) and CLock Polarity (0/1) are used to describe SPI. |
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beitman
63 Posts |
Posted - 01 Oct 2007 : 17:19:40
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You are correct, those names are unfortunate interpretations of the descriptions in the EP915 manual. You may also use (0/1) as these values are written to the correct bits in the SSPCR0 register.
LEADING/TRAILING would be better as FIRST_EDGE/SECOND_EDGE.
POSITIVE/NEGATIVE would be better as LOW/HIGH |
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bkohan
2 Posts |
Posted - 02 Oct 2007 : 07:39:12
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quote:
[i]Originally posted by beitman[/i]
[br]You are correct, those names are unfortunate interpretations of the descriptions in the EP915 manual. You may also use (0/1) as these values are written to the correct bits in the SSPCR0 register.
LEADING/TRAILING would be better as FIRST_EDGE/SECOND_EDGE.
POSITIVE/NEGATIVE would be better as LOW/HIGH
So, based on the code snippet, what does the clock look like and when is the data "clocked"? In SPI lingo a clock polarity of low/negative would mean the clock is Low when not active. |
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beitman
63 Posts |
Posted - 02 Oct 2007 : 16:40:17
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sspConfig.sspClkPolarity is used to set the SPO bit in the SSPCR0 register of the SSP controller on the EP9315. The description of that bit from the EP9315 Users Guide is:
"When the SPO clock polarity control bit is LOW, it produces a steady state low value on SCLKOUT pin. If the SPO clock polarity control bit is HIGH, a steady state high value is place on the SCLKOUT pin when data is not being transferred."
Per the EP9315 Users Guide, for Motorola SPI protocol, you would set
sspConfig.sspClkPhase = 0;
sspConfig.sspClkPolarity = 0;
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