General advice - first time programmer

reelthymes · Joined: 26 Jul 2024 Posts: 2

Hi,

I have tested embedded systems before but this is my first time developing firmware itself. I have a few questions. I have a device that already has an existing firmware codebase, I am just trying to take that code and make some modifications in order to add an additional functionality. This device is very simple, it is basically a 1P2T switch and it can be controlled by USB, as well as daisy chained as a slave device on both SPI and I2C interfaces. Here is some additional info of my device and environment:

MPLAB v8.92
PCD compiler
MCU is PIC24FJ256GB206

This is what I'm trying to do:

In the legacy code, the I2C interface of the device was used so that the device can be a slave device. For this revision, I will get rid of the I2C bus ( I believe it's defined in the SW using functions) and instead I will use the 3 lines that used to be used for the I2C address bits as lines that are inputs from an external push button. So basically the switch can be controlled via an external push button.

I noticed that the original code had a few lines that enabled interrupts:

Ttelmah · Joined: 11 Mar 2010 Posts: 19336

You really need to read the chip data sheet.
_each interrupt has two enable bits that control whether it is accepted+.
It's own interrupt enable, and a global enable.
Both have to ve set before an interrupt will be handled.
The 'point' of the global bit is if you want to disable all the interrupts, this
can be disabled, rather than having to turn them all off individually.

On the pull-ups, this depends on the hardware. You need to look at
how the lines are wired. and how they are driven, So if the lines are
driven low, or have external connections they won't want pullups.
it actually uses fractionally less power to drive unused lines low, than
to enable the pullups.

reelthymes · Joined: 26 Jul 2024 Posts: 2

Thanks for the info. I have been reading the Microchip's datasheet for the PIC24FJ256GB210 Family of devices. I am still learning what some of the lingo means, I also have the book "making embedded systems" which is helping. I will keep reading through the datasheet.

Ttelmah · Joined: 11 Mar 2010 Posts: 19336

Good.
Key is a bit of history. On the older PIC's (12/14/16/18), there is an actual
global interrupt enable bit. On the 16bit PIC's, this is simulated by changing
the priority control. So setting the top three bits of the SRL, disables all the
normal interrupts (but not traps). While clearing these bits enables all
the interrupts/ The compiler retains the 'old' instruction, and using this
just turns the priority up and down (enable/disable).