Reading data-sheets is not a fun task. Going through tens of hundreds of pages with numerical, mathematical and scientific data is not fun reading. This blog post attempts to simplify reading the available data-sheets related to PIC24 micro-controller used in the PSLab device to help reader with implementing a new feature in PSLab firmware.
There are many features available in the PSLab device, such as; UART, SPI, I2C, ADC and Basic I/O reading. The “basic” implementation techniques do not vary much from one feature to other. That being stated this blog will carry out the basic implementation techniques one should follow and basic knowledge on PIC micro-controller programming to save himself from the trouble going through the 500+ pages in PIC data-sheets.
PIC Basics:
Before go into implementation there are few facts one should know about PIC programming.
– In the micro-controller values are saved in a memory block known as Registers. The values saved in these registers are volatile as they are all set to 0 regardless the value they were assigned when the power is off.
– Micro-controller configurations are made by setting values to these registers. Even turning on and off a whole feature like UART in PSLab device can be done using setting 0 to UARTEN register bit.
– When it comes to I/O ports, there are two different types of registers called TRIS and LAT/PORT. By setting 1 to TRIS ports will make the relevant pin an input pin. Setting it to 0 will make it an output pin. Easy way to remember this is think 1 as I in input and 0 as O in output. In UART implementation of PSLab, pin RP40 is set as an input pin to receive the data stream and pin RP39 is set as an output pin to send the data stream out. These settings are made using TRIS port settings. PORT registers save the value received by the relevant input pin attached to it.
The above figure extracted from mikroe learning materials, illustrates different stages an I/O pin can handle. As an extra point, ANSEL register makes the pin support digital signals or analog signals as per user requirements.
– In PIC, some registers such as PORT, TRIS and registers with similar functionalities are combined together. To access the value of each individual register can be done using dot notation. Assume the program requires to access the 8th register in TRISB register set. Note that the registers are indexed from zero. This implies that the 8th register will have the index 8 in the register sub-set. The following syntax is used to access the register;
TRISBbits.TRISB7
The above points cover the basic knowledge one should have when developing firmware to PSLab device.
How to implement a feature like UART in PSLab firmware?
The first thing to know when implementing a new feature is that the developer needs to be familiar with the relevant hardware protocols. As an example, to implement UART; relevant protocol is RS232. If the feature is I2C; one should know about the I2C protocol.
Once the feature is familiar, next step is to refer the PIC data-sheet and resources on how to implement it in firmware. As for demonstrative purposes, this blog will continue with UART implementation.
Download the latest data-sheet from MicroChip official website and browse to the table of content. It consists of a set of features supported by the micro-controller implemented in the PSLab device. Find the entry related to the feature being implemented. In this case it will be Universal Asynchronous Receiver Transmitter (UART).
Each feature will contain a description following this format explaining what are the options it support and its constraints.
One must be aware of the fact that not every pin in the micro-controller can be used for any feature as he desires. The “PINOUT I/O DESCRIPTIONS” section in the data-sheet explains which pins are capable of the feature being implemented. According to these details, the pins should be initiated as Input/Output pins as well as Digital/Analog pins.
The next step is to refer to the control registers related to the feature. They are all mentioned in the data-sheet under the specific feature. There are some notations available in this section which resembles something like the following;
PDSEL<1:0>: Parity and Data Selection bits 11 = 9-bit data, no parity 10 = 8-bit data, odd parity 01 = 8-bit data, even parity 00 = 8-bit data, no parity
This represents a register with two bits. By setting either 11 or 10 or 01 or 00; different implementations can be achieved.
In PSLab firmware this is implemented as;
U1MODEbits.PDSEL = 0;
which implements UART feature with 8-bits stream having no parity bits for error correction.
In UART feature implemented in PSLab device, receiving bit stream is fetched by reading the register values in U1STAbits.URXDA and data is transmitted using U1TXREG. All these registers are mentioned in the control registers section in the feature.
Resources:
- I/O Ports: https://learn.mikroe.com/ebooks/picmicrocontrollersprogramminginassembly/chapter/io-ports/
- RS232 Protocol:http://extremeelectronics.co.in/avr-tutorials/rs232-communication-the-basics/
- I2C Protocol: https://learn.sparkfun.com/tutorials/i2c
- PIC24EP64GP204 Datasheet: http://pdf1.alldatasheet.com/datasheet-pdf/view/545636/MICROCHIP/PIC24EP64GP204.html
- PSLab firmware Repository: https://github.com/fossasia/pslab-firmware