PQDB and gpboard in PICSimLab: PIC18F4520 and the gpsim Board
This page pairs two of PICSimLab’s least typical PIC boards. The PQDB is an open-source hardware board adapted to a single PIC chip, while the gpboard is the only board in PICSimLab that runs on the gpsim backend instead of picsim — a difference that has one important consequence for debugging, stated plainly below.
Overview
The PQDB is an open hardware board that was originally designed for Arduino/freedom-style boards; in PICSimLab it is adapted to run the PIC18F4520 on the picsim backend. The gpboard is a generic board — only reset, serial and crystal circuits — backed by gpsim, the other PIC simulator PICSimLab can use.
Supported MCUs
| Board | Microcontroller | Backend | Debugging |
|---|---|---|---|
| PQDB | PIC18F4520 | picsim | MPLAB X Integrated Debug |
| gpboard | Generic PIC board (gpsim) | gpsim | None yet |
Backend & debugging
The PQDB behaves like any other picsim board: MPLAB X Integrated Debug through the PICSimLab plugin (see the plugin guide), and MPLAB X + XC8 projects from the Code Project Wizard.
The gpboard is different, and this is the fact to remember: the official documentation lists the debugging support for the gpsim backend as “none yet”. You can run code on the gpboard, but there is currently no MPLAB X or GDB debugging path for it in PICSimLab. Any source that claims you can debug a gpboard project the way you debug a picsim board is wrong.
What you can do
- Run PIC18F4520 projects on the PQDB — the chip is also on the McLab2 and PICGenios lists, so general 18F workflow material such as the PIC18F4550 first project (project generation, XC8 build, hex loading) carries over.
- Use the gpboard when you specifically want gpsim’s simulation of a generic PIC circuit built from spare parts, accepting run-only operation.
- On either board, add displays, inputs and instruments from Modules → Spare parts and the oscilloscope module.
Real limits
- gpboard debugging: none yet. If stepping through code matters, choose a picsim board instead — that is the honest recommendation.
- The PQDB adaptation is a single-chip board in PICSimLab (PIC18F4520). Its open-hardware origins do not mean other chips from the original board family are selectable here.
- The gpboard is generic by design: reset, serial and crystal only — everything else is spare parts you add.
- picsim’s documented peripheral coverage (ADC, interrupts, WDT, TMR0–TMR2, USART, CCP, EEPROM, I/O ports) bounds what the PQDB can model.
Both boards ship with PICSimLab itself — install it from the download page and select PQDB or gpboard from the board list.