STV3550 datasheet(101/144 Pages) STMICROELECTRONICS

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STV3550

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At a CPU clock frequency of 40 MHz, the following baud rates are generated as shown in Table 23.

Note:

The reload value must be greater than zero and it will also have a minimum value (i.e. maximum

frequency) which is related to the CPU clock frequency. In normal mode this should be a minimum

of 0x0008. In I²C mode it is the sum of the settings of the data_hold constant and the data_setup

constant. i.e. 21 + 18 = 0x0027 at 60 MHz or 7 + 6 = 0x000D at 20 MHz (see Table 25).

The value in SSCBRG is used to load a counter at the start of each clock cycle. The counter counts

down until it reaches 1 and then flips the clock to the opposite logic value. Consequently, the clock

produced is twice the SSCBRG number of CPU clock cycles.

Reading the SSCBRG register will return the current count value. This can be used to determine

how far into each half cycle the counter is.

7.7.6

Shift Register

The shift register is loaded at the start of a data frame with the data in the transmit buffer. It then

shifts data out of the serial_data_out port and data in from the serial_data_in port.

The shift register can shift out LSB first or MSB first. This is programmed by the heading control bit,

SSCHB in the control register. A logic 1 indicates that the MSB will be shifted out first and a logic 0

that the LSB will shift first.

The width of a data frame is also programmable from 2 bits to 16 bits. This is set by the SSCBM bit

field of the control register. A value of “0000” is not allowed, subsequent values set the bit width to

the value plus one e.g. “0001” sets the frame width to 2 bits and “1111” sets it to 16 bits.

When shifting LSB first, data comes into the shift register at the MSB of the programmed frame

width and is taken out of the LSB of the register. When shifting in MSB first, data is placed into the

Table 23: Baud Rates and Bit Times for Various SSCBRG Reload Values

Baud Rate

Bit Time

Reload Value

Reserved. Use a

reload value > 0

0000H

5 MBaud

200 ns

0004H

3.3 MBaud

300 ns

0006H

2.5 MBaud

400 ns

0008H

2.0 MBaud

500 ns

000AH

1.0 MBaud

1 us

0014H

100 KBaud

10 us

00C8H

10 KBaud

100 us

07D0H

1.0 KBaud

1 ms

4E20H

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