Nuts and Volts - April 2014

MCP9700A vs. DS18B20

We’re using the 9700A primarily because it provides the opportunity to experiment with sending analog values from the PICAXE to the Pi. However, if your main interest is in the temperature measurement itself, there are these three good reasons to choose the MCP9700A rather than the DS18B20:

1. The DS18B20 is at least 10 times more expensive than the 9700A.

2. A PICAXE processor takes as long as 750 mS to fetch the digital temperature data from a DS18B20, but it only takes about 1.25 mS to read an analog input. In some projects, this huge difference really isn’t very significant, but we ultimately want to program the Pi to interrupt the PICAXE whenever it (Pi) wants updated data.

As you may remember, the PICAXE checks for an interrupt condition after the completion of every instruction (and frequently as wait or pause commands are executed).

Consequently, if the PICAXE happens to be in the process of reading data from a 18B20 sensor when an interrupt occurs, it could take as long as 750 mS for it to respond, which would certainly complicate the Pi programming.

3. Finally, since the DS18B20 is a digital device, the Pi doesn’t need a PICAXE to interface with it; it can do so all by itself! If you’re interested in that approach, you can search for “Raspberry Pi DS18B20” (without the quotes) — you will find many relevant projects. (Also, you might want to check out #11 in the Adafruit.com

Raspberry Pi Tutorials.)

We’re going to conduct three different experiments this month, and all of them can be carried out with the same hardware setup, so let’s begin with that. Figure 1 presents the schematic of the circuit that we’ll use for our experiments.

The first thing to note is that I haven’t included the PICAXE programming connections to the 08M2 pins C.0 (SerOut) and C. 5 (SerIn) but, of course, they are required!

Also note that Vcc = 3.3V, which is the supply voltage for all our PICAXE-Pi projects. (In case you’re wondering, the 9700A’s supply range is 2.3V to 5.5V, so it will work fine in our 3.3V circuit.)

Finally, we won’t be using the connection between GPIO 14 (TxD) on the Pi and pin C. 2 on the 08M2. I only included it in case you (or I) want to conduct additional experiments with sending serial data from the Pi to the PICAXE.

Figure 2 is a photo of my breadboard setup for this month’s experiments. As you can see, I’m again using the stripboard interface circuit that we constructed in our first PICAXE-Pi article (August 2013).

Of course, you can use any hardware setup you prefer — just make sure that the 08M2 is powered at 3.3V to match the levels on the Pi’s GPIO pins, and that there is a series resistor in the I/O connections that we are using. (The stripboard interface circuit includes 470 W resistors in each GPIO line.)

If you are using the stripboard circuit, you may want to refer to Figure 3 which presents the pinout of the GPIO header on the stripboard circuit.

PICAXE-Pi Communications — Part 2

In this month's Primer, we're going to continue our PICAXE-Pi serial communication experiments. As you probably already know, there aren't any ADC (analog-to-digital converter) inputs available on the Pi, but PICAXE processors have plenty of them: three on the 08M2; seven on the 14M2; 10 on the 18M2; and 11 on the

20M2 and 20X2. To demonstrate one way that we can transfer an ADC reading from a PICAXE processor to the Pi, we're going to interface an MCP9700A (a.k.a., 9700A) analog temperature sensor with an 08M2 processor, and serially send the sensor's ADC reading on to the Pi for display. Of course, we can also do the same thing with any PICAXE processor, and with a variety of analog sensors. By the time we complete this month's experiments, we will have covered the basic techniques that can be used to send a PICAXE ADC reading to the Pi, from any analog sensor we choose. So, let's get started!