PC912XDP512F0MPVR datasheet(535/1348 Pages) FREESCALE

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Chapter 12 Serial Peripheral Interface (S12SPIV4)

MC9S12XDP512 Data Sheet, Rev. 2.21

Freescale Semiconductor

535

12.4.4

SPI Baud Rate Generation

Baud rate generation consists of a series of divider stages. Six bits in the SPI baud rate register (SPPR2,

SPPR1, SPPR0, SPR2, SPR1, and SPR0) determine the divisor to the SPI module clock which results in

the SPI baud rate.

The SPI clock rate is determined by the product of the value in the baud rate preselection bits

(SPPR2–SPPR0) and the value in the baud rate selection bits (SPR2–SPR0). The module clock divisor

equation is shown in Equation 12-3.

BaudRateDivisor = (SPPR + 1)

• 2(SPR + 1)

Eqn. 12-3

When all bits are clear (the default condition), the SPI module clock is divided by 2. When the selection

bits (SPR2–SPR0) are 001 and the preselection bits (SPPR2–SPPR0) are 000, the module clock divisor

becomes 4. When the selection bits are 010, the module clock divisor becomes 8, etc.

When the preselection bits are 001, the divisor determined by the selection bits is multiplied by 2. When

the preselection bits are 010, the divisor is multiplied by 3, etc. See Table 12-6 for baud rate calculations

for all bit conditions, based on a 25 MHz bus clock. The two sets of selects allows the clock to be divided

by a non-power of two to achieve other baud rates such as divide by 6, divide by 10, etc.

The baud rate generator is activated only when the SPI is in master mode and a serial transfer is taking

place. In the other cases, the divider is disabled to decrease IDD current.

NOTE

For maximum allowed baud rates, please refer to the SPI Electrical

Speciﬁcation in the Electricals chapter of this data sheet.

12.4.5

Special Features

12.4.5.1

SS Output

The SS output feature automatically drives the SS pin low during transmission to select external devices

and drives it high during idle to deselect external devices. When SS output is selected, the SS output pin

is connected to the SS input pin of the external device.

The SS output is available only in master mode during normal SPI operation by asserting SSOE and

MODFEN bit as shown in Table 12-2.

The mode fault feature is disabled while SS output is enabled.

NOTE

Care must be taken when using the SS output feature in a multimaster

system because the mode fault feature is not available for detecting system

errors between masters.

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