18X download and reset circuits

[pme720]

I know the answers are staring at me in the volumes of information on this website, but they are not written in such a way that I recognise them. As a double check, I ask:

If I build my own project with no need of a reset switch or a download circuit (I download my program via axe090 then mount 18X) I need to connect serin to 0V and reset (pin 4) to +V. OK. Do I need to use resistors for this, and if so what size? 22K for serin and 4.7K for pin 4? 10K + 22K for serin and anything from 1K to 10K for pin 4? Or are no resistors necessary? Thanks

 

[womai]

You don't need resistors in your case. Just tie the serin to GND and RESET to Vcc (power). Though I tend to find it useful to preserve at least the download option by using at least a jumper rather than a hardwired ground connection.

The reason the "normal" Picaxe circuitry has those resistors is so the serial input can pull up the input line (and the reset switch can pull down the reset line) without creating a short circuit.

Another reason to use resistors rather than a straight connection is when the pin in question can also become an output. This could be during startup (before the Pixaxe gets into a well defined state) or because of a programming bug (e.g. driving the wrong pin). If you tie the pin to ground and then drive a one (high), the output may get destroyed because of excessive current flow. A resistor still pulls it low, but limits the maximum current to a small value.

Wolfgang

 

[ylp88]

It is probably best practice to use resistors. Using resistors will also lower the power consumption of your circuit which becomes important for portable devices and circuits powered from solar cells etc...

4.7k is common for the reset pin. 10k + 22k = 32k so something around there should do fine.

<b><i>ylp88 </b> </i>

 

[hippy]

For the 16F88 (PICAXE-18X), and probably for other PICmicro devices, Microchip recommend that MCLR (Reset) is not tied directly to +V. From the datasheet ...

&quot;The behavior of the ESD protection on the MCLR pin has been altered from previous devices of this family. Voltages applied to the pin that exceed its specification
can result in both MCLR and excessive current beyond the device specification during the ESD event. The circuit, as shown in Figure 15-2, is suggested.

Note: For this reason, Microchip recommends
that the MCLR pin no longer be tied
directly to VDD.&quot;