Copy Firmware out of Microchip PIC16F873

The master controller (Processor 1) initiates the data transfer by sending the SCK signal. Data is shifted out of both shift registers on their programmed clock edge and latched on the opposite edge of the clock when copy firmware from microchip chip.

Both processors should be programmed to the same Clock Polarity (CKP), then both controllers would send and receive data at the same time. Whether the data is meaningful (or dummy data) depends on the application software.

This leads to three scenarios for data transmission:
• Master sends data – Slave sends dummy data
• Master sends data – Slave sends data
• Master sends dummy data – Slave sends data before copy the microchip chip program

The master can initiate the data transfer at any time because it controls the SCK. The master determines when the slave (Processor 2, Figure 14-2) is to broadcast data by the software protocol if the program of microchip chip being copied.

In Master mode, the data is transmitted/received as soon as the SSPBUF register is written to. If the SPI 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” mode. The clock polarity is selected by appropriately programming the CKP bit (SSPCON<4>) after the firmware being copied.

This then, would give waveforms for SPI communication as shown in  Figure 14-3, Figure 14-5 and Figure 14-6, where the MSB is transmitted first. In Master mode, the SPI clock rate (bit rate) is user programmable to be one of the following:

FOSC/4 (or TCY)

FOSC/16 (or 4 · TCY)

FOSC/64 (or 16 · TCY)

Timer2 output/2

This allows a maximum data rate (at 40 MHz) of 10 Mbps. Figure 14-3 shows the waveforms for Master mode. When the CKE bit is set, 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.