Pic Serial Programmer

Posted : admin On 12/25/2021

PIC-PG3 is programmer based on D.Tait's parallel port design. To operate it needs external power supply 12-15VAC or 16-18VDC. It supports all 8/18/28 and 40 pin PIC microcontrollers which allow serial programming. PIC-PG3 have connector and cable for ICSP programming and can be used to program all PIC-PxxB prototype boards.

A while back I bought a couple of PIC16F57 (DIP) chips because they were dirt cheap. I figured someday I could use these in something. Yes, I know, this is a horrible way to actually build something and a great way to accumulate junk. However, this time the bet paid off! Only about a year or two too late; but that's beside the point. The problem I now had was that I didn't have a PIC programmer.

When I bought these chips I figured I could easily rig a board up to the chip via USB. Lo and behold, I didn't read the docs properly; this chipset doesn't have a USB to serial interface. Instead, it only supports Microchip's In-Circuit Serial Programming (ICSP) protocol via direct serial communication. Rather than spend the $40 to buy a PIC programmer (thus, accumulating even more junk I don't need), I decided to think about how I could make this happen.

Glancing at some of my extra devices lying around, I noticed an unused Arduino. This is how the idea for this project came to life. Believe me, the irony of programming a PIC chip with an ATMega is not lost on me. So for all of you asking, 'why would anyone do this?' the answer is two-fold. First, I didn't want to accumulate even more electronics I would not use often. Second, these exercises are just fun from time to time!

Hardware Design

Pic Serial Programmer

My prototype's hardware design is targeted to using an Arduino Uno (rev 3) and a PIC16F57. Assuming the protocol looks the same for other ICSP devices, a more reusable platform could emerge from a common connector interface. Likewise, for other one-offs it could easily be adapted for different pinouts. Today, however, I just have the direct design for interfacing these two devices:

Overall, the design can't get much simpler. For power I have two voltage sources. The Arduino is USB-powered and the 5V output powers the PIC chip. Similarly, I have a separate +12V source for entering/exiting PIC programming mode. For communication, I have tied the serial communication pins from the Arduino directly to the PIC device.

The most complicated portion of this design is the transistor configuration; though even this is straightforward. I use the transistor to switch the 12V supply to the PIC chip. If I drive the Arduino pin 13 high, the 12V source shunts to ground. Otherwise, 12V is supplied to the MCLR pin on the PIC chip. I make no claims that this is the most efficient design (either via layout or power consumption), but it's my first working prototype.

Serial Communication with an Arduino

Arduino has made serial communication pretty trivial. The only problem is that the Arduino's serial communication ports are UART. That is to say, the serial communication is asynchronous. The specification for programming a PIC chip with ICSP clearly states a need for a highly controlled clock for synchronous serial communication. This means that the Arduino's Serial interface won't work for us. As a result, we will go on to use the Arduino to generate our own serial clock and also signal the data bits accordingly.

Setting the Clock Speed

The first task to managing our own serial communication with the Arduino is to select an appropriate clock speed. The key to choosing this speed was finding a suitable trade-off between programming speed (i.e. fast baud rate) vs. computation speed on the Arduino (i.e. cycles of computation between each clock tick).

Remember, the Arduino is ultimately running an infinite loop and isn't actually doing any parallel computation. This means that the amount of time it takes to perform all of your logic for switching data bits must be negligible between clock ticks. If your computation time is longer than or close to the clock ticking frequency, the computation will actually impact the clock's ability to tick steadily. As a rule of thumb, you can always set your clock rate to have a period that is roughly 1 to 2 orders of magnitude than your total computation.

Microchip Pic Programmer

Taking these factors into account, I chose 9600 baud (or a clock at 9.6KHz). To perform all the logic required for sending the appropriate programming data bits, I estimated somewhere in the 100's of nanoseconds to 1's of microseconds for computation. Giving myself some headroom, I selected a standard baud rate that was roughly two orders of magnitude larger than my computation estimate. Namely, a period of 104 microseconds corresponds to a 9.6KHz clock.

After completing the project I could have optimized my clock speed. However, that was unnecessary for this project. The clock rate I had selected worked well. The 9600 baud rate is fast enough for timely programming the device because we don't have much data to transmit. Similarly, it provides us a lot of headroom to experiment with different types of computation.

Generating the Clock Signal

While this discussion has primarily focused on the design decisions involved in choosing a clock signal rate, how did we generate it? The process really comes down to toggling a GPIO pin on the Arduino. In our specific implementation, I chose pin 2 on the Arduino. While you can refer to the code for more specific details, an outline of this process follows: