Nuts and Volts - July 2014

The problem seemed to be in the initial wake-up speed of the PIC32MX’s SPI clock. I lowered the SPI clock speed from 10 MHz to 4 MHz, which solved the “dead display”

problem. Here’s the updated SPI configuration code that runs in the init() function:

■ Screenshot 2. Just in case you're not from

 {
 SPI2BUF = 0x00;
 //send 0x00
 while(SPI2STATbits.
 SPIRBF == 0);
 //wait until bus
 //cycle complete
 dummybite = SPI2BUF;
 while(SPI2STATbits.
 SPIBUSY == 1);
 }
 CShi;
}
 SPI2CONbits.ON = 0;
 //disable SPI2 peripheral
 SPI2CONbits.MSTEN = 1; //Master mode
 SPI2CONbits.CKP = 0;
 //polarity idle low
 SPI2CONbits.CKE = 1;
 //transmit on active to idle
 SPI2CONbits.SMP = 0;
 //sample in the middle
 SPI2BRG = 0x13;
 //4MHz SPI clock
 SPI2STATbits.SPIROV = 0;
 //clear overflow flag
 SPI2CONbits.ON = 1;
 //enable SPI2 peripheral

Tennessee, high cotton means tall cotton — and we seem to be walking in it.

Waking the FT800

Table 5 lists the commands that we can transfer and execute using the hostCmd function.

SPI Setup

Before we can access the FT800, we must wake it up. Naturally, before we can throw that bucket of cold water on it, we must execute the init() function to set up the PIC32MX’s GPIO, clock, and SPI portal. The PIC32MX init() function also insures that the CS_N and PD_N (Power Down) pins are forced to their inactive states.

There are a couple of user-accessable LEDs on the MX3 that are initialized. If you are new to PIC32MX programming, one of the most important gotchas to look out for is the JTAG interface which is enabled by default:

While I was chewing on a piece of elephant, I noticed that my original SPI setup was not working as designed.

#define CSlo LATGCLR = 0x0200;
 //0000 0010 0000 0000
#define CShi LATGSET = 0x0200;
 
#define PDlo LATBCLR = 0x0020;
 //0000 0000 0010 0000
 #define PDhi LATBSET = 0x0020;
 
 #define LD4on LATFSET = 0x0001;
 //0000 0000 0000 0001
 #define LD4off  LATFCLR = 0x0001;
 #define LD4tog  LATFINV = 0x0001;
 #define LD5on LATFSET = 0x0002;
 //0000 0000 0000 0010
 #define LD5off  LATFCLR = 0x0002;
 
 CShi; //SPI slave select inactive
 PDhi;  //power down inactive
 LD4off;  //utility LEDs on MX3
 LD5off;
 DDPCONbits.JTAGEN = 0;

I included the lo-hi-on-off mini macros for clarity. Using the PIC32MX atomic I/O operations eliminates the need for a schematic just to identify these particular I/O pin assignments. The FT800 wake-up sequence begins by driving the FT800’s PD pin logically low for 20 mS, and then driving the PD pin logically high for 20 mS:

■ Table 6. I figured I had nothing to lose, so I plugged these values into the driver. As it turned out, I was able to continue walking in the cotton field.