Rainbow-electronics AT89C5131 Bedienungsanleitung PDF herunterladen (Seite 105)

105

AT89C5131

4136A–USB–03/03

pins (Figure 42). To prevent bus conflicts on the MISO line, only one slave should be

selected at a time by the Master for a transmission.

In a Master configuration, the SS

line can be used in conjunction with the MODF flag in

the SPI Status register (SPSTA) to prevent multiple masters from driving MOSI and

SCK (see Section “Error Conditions”, page 109).

A high level on the SS

pin puts the MISO line of a Slave SPI in a high-impedance state.

The SS

pin could be used as a general-purpose if the following conditions are met:

• The device is configured as a Master and the SSDIS control bit in SPCON is set.

This kind of configuration can be found when only one Master is driving the network

and there is no way that the SS

pin could be pulled low. Therefore, the MODF flag in

the SPSTA will never be set

(1)

• The Device is configured as a Slave with CPHA and SSDIS control bits set

(2)

This

kind of configuration can happen when the system comprises one Master and one

Slave only. Therefore, the device should always be selected and there is no reason

that the Master uses the SS

pin to select the communicating Slave device.

Notes: 1. Clearing SSDIS control bit does not clear MODF.

2. Special care should be taken not to set SSDIS control bit when CPHA =’0’ because in

this mode, the SS

is used to start the transmission.

Baud Rate In Master mode, the baud rate can be selected from a baud rate generator which is con-

trolled by three bits in the SPCON register: SPR2, SPR1 and SPR0. The Master clock is

chosen from one of seven clock rates resulting from the division of the internal clock by

2, 4, 8, 16, 32, 64 or 128.

Table 76 gives the different clock rates selected by SPR2:SPR1:SPR0:

Table 76. SPI Master Baud Rate Selection

SPR2 SPR1 SPR0 Clock Rate Baud Rate Divisor (BD)

000 Don’t Use No BRG

001 F

CLK PERIPH

/4 4

010 F

CLK PERIPH

/8 8

011 F

CLK PERIPH

/16 16

100 F

CLK PERIPH

/32 32

101 F

CLK PERIPH

/64 64

110 F

CLK PERIPH

/128 128

111 Don’t Use No BRG

1 2 ... 100 101 102 103 104 105 106 107 108 109 110 ... 174 175

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