Nuts and Volts - August 2015

In my opinion, many boards like our board have too many pushbuttons. The sole intent of designing in the single pushbutton switch was bootloading control. My first use of the pushbutton switch during development of the code for this column was in a debugging sequence. So, it’s there if we need it for whatever we may need it for.

The PIC32MX795F512L is programmed and debugged using a PICkit 3, which is attached to our board’s male five-pin ICSP connector using a six inch cable assembly terminated by a five-pin DF13 1.25 mm female header.

USB in a Chip

The FT232RL is attached to the PIC32MX795F512L via its transmit and receive lines. This module will most likely be used to communicate with a host USB device such as a laptop PC.

The only physical housekeeping we have to do here is make sure we don’t over-current the indicator LEDs. The real work is in the firmware driver. You can view the FT232RL hardware details in Schematic 2.

RS-232 is Not Dead

The news of the death of RS- 232 has been greatly exaggerated. RS-232 interface ICs are not hard to find, and many modern devices are equipped with a legacy RS-Like the USB subsystem, the RS-232 subsystem is not a burden on the hardware side of the fence. According to Schematic 3, we only need to add five 100 nF capacitors to the mix.

microSD on a String

A slightly modified version of SparkFun’s OpenLog is drawn up in Schematic 4. The original intent of the OpenLog was to simply store the incoming stream of RS- 232 data on the microSD card. However, there are commands associated with the OpenLog that allow us to control the microSD card file system.

So, we can choose to use this microSD card to log data or store data under program control. The really neat thing about it all is that the control and data transfers are done via one of the PIC32MX795F512L’s UARTs.

microSD the Microchip Way

The “other” microSD card is being fed from one of the PIC32MX795F512L’s SPI portals. Like the OpenLog microSD card, the “other” microSD card can be used to log data or store data under program control using Microchip’s microSD card file system.

However, the microSD card under the control of the PIC32 also has the ability to hold firmware images that can be bootloaded into the PIC32MX795F512L. Jumper blocks are used to set the card

62 August 2015

■ Schematic 3. Setting up an RS-232 hardware port used to be a real pain.

I remember back in the day having to lash up separate receiver and transmitter ICs that required negative supply voltages. Today's RS-232 interface ICs are a breeze to use.