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PIC16LC771 数据表(PDF) 72 Page - Microchip Technology |
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PIC16LC771 数据表(HTML) 72 Page - Microchip Technology |
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72 / 224 page  PIC16C717/770/771 DS41120C-page 72 Advance Information 1999-2013 Microchip Technology Inc. 9.1.4 MASTER MODE The master can initiate the data transfer at any time because it controls the SCK. The master determines when the slave (Processor 2, Figure 9-2) is to broad- cast data by the software protocol. In Master mode, the data is transmitted/received as soon as the SSPBUF register is written to. If the SPI module is only going to receive, the SDO output could be disabled (programmed as an input). The SSPSR register will continue to shift in the signal present on the SDI pin at the programmed clock rate. As each byte is received, it will be loaded into the SSPBUF register as if a normal received byte (interrupts and status bits appropriately set). This could be useful in receiver applications as a “line activity monitor”. The clock polarity is selected by appropriately program- ming bit CKP (SSPCON<4>). This then would give waveforms for SPI communication as shown in Figure 9-3, Figure 9-5 and Figure 9-6, where the MSb is transmitted first. In Master mode, the SPI clock rate (bit rate) is user programmable to be one of the follow- ing: •FOSC/4 (or TCY) •FOSC/16 (or 4 • TCY) •FOSC/64 (or 16 • TCY) • Timer2 output/2 This allows a maximum bit clock frequency (at 20 MHz) of 8.25 MHz. Figure 9-3 shows the waveforms for Master mode. When CKE = 1, the SDO data is valid before there is a clock edge on SCK. The change of the input sample is shown based on the state of the SMP bit. The time when the SSPBUF is loaded with the received data is shown. FIGURE 9-3: SPI MODE WAVEFORM (MASTER MODE) SCK (CKP = 0 SCK (CKP = 1 SCK (CKP = 0 SCK (CKP = 1 4 Clock modes Input Sample Input Sample SDI bit7 bit0 SDO bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 bit7 bit0 SDI SSPIF (SMP = 1) (SMP = 0) (SMP = 1) CKE = 1) CKE = 0) CKE = 1) CKE = 0) (SMP = 0) Write to SSPBUF SSPSR to SSPBUF SDO bit7 bit6 bit5 bit4 bit3 bit2 bit1 bit0 (CKE = 0) (CKE = 1) Next Q4 cycle after Q2 |
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