Cornell University ECE4760
Time Measurement and Control
plus clock generation
PIC32MX250F128B

The larger goal is to use parallel hardware to make timing completely independent of software, including independent of interrupts.

Time measurement and control
There are eight hardware timers on the PIC32MX250. Each of them is an independent counter which is toggled by one of several clock sources and which may trigger interrupts or other hardware events such as DMA bursts, ADC conversions, or pulse trains. The timers are not equivalent. Some have specific hardware abilities that the others do not. See also the Hardware Manual. There are two other functions which are closely related to timers: Input Capture which uses the timers to measure precise intervals, and Output Compare which use a timer to produce precise output pulse trains such as PWM and variable frequency square waves. There are five input capture units and five output compare units, each of which may be linked to either timer 2 or timer 3. This page also talks a little about the available time-base oscillators the PIC32 can generate. An example in an older lab uses one timer to drive two output compare units to produce two pulse trains, and a second timer to act a source for input capture of the period of either pulse train.

Timers

Input Capture Units
Timer 2 or 3 can be used as a time source for time capture based on an external event, such as a rising or falling edge on an ICx pin.. This allows very accurate (1-cycle) timing of events independent of software. Our cpu has five input capture units. All if the IC event-inputs are on PPS, and therefore may be moved between 8 different external pins. An example in An older lab uses one timer to drive two output compare units to produce two pulse trains, and a second timer to act a source for input capture of the period of either pulse train.



Output Compare Units
Timer 2 or 3 can be used as a time source for generating an external event, such as a rising or falling edge or PWM signal. This allows very accurate (1-cycle) generation of events independent of software. Our cpu has five output compare units. All if the OCx pulse-outputs are on PPS, and therefore may be moved between 8 different external pins. There is Hardware-based PWM Fault detection and automatic output disable (OCFA or OCFB inputs). An example in An older lab uses one timer to drive two output compare units to produce two pulse trains, and a second timer to act a source for input capture of the period of either pulse train.

Oscillators
The PIC32 oscillator system has the following modules and features:
• Four external and internal oscillator options as clock sources. Primary osc, secondary osc, fast RC osc, low power RC osc.
• On-chip Phase-Locked Loop (PLL) with a user-selectable input divider and multiplier, as well as an output divider, to boost operating frequency on some internal and external oscillator sources
• On-chip user-selectable divisor postscaler on some oscillator sources

Generally in this class, the oscillator will default to the fast RC, FRC, 8 MHz +/-0.9%, clock, as specified in the Protothreads config file.
See the full list of configuration-bit options for more information.
In the file there are three pragma lines:
#pragma config FNOSC = FRCPLL, POSCMOD = OFF
#pragma config FPLLIDIV = DIV_2, FPLLMUL = MUL_20, FPLLODIV = DIV_2 //40 MHz
#pragma config FPBDIV = DIV_1 // PB 40 MHz
--The first line sets the Oscillator Selection Bits (FNOSC) to use the Fast RC Osc with PLL. The PLL allows the system to multiply the 8 MHz FRC to a higher rate. It also disables the primary oscillator by setting POSCMOD off. See below for using the primary oscillator.
--The second line divides the FRC by two because the PLL input must be 4 MHz. Then it sets the PLL to multiply by 20 and divide by two to get 40 MHz cpu clock. Range is 15≤FPLLMUL≤24 and 1≤FPLLODIV≤256. But check the config page for details.
--The third line sets the peripherial bus speed to be the same as the cpu speed. FPBDIV can be set to 1,2,4,8.

From the diagram below (figure 6-1), you can see that there are many ways of setting up the oscillator system to minimize power, or maximize performance, or run USB. To run the USB PLL you must use the primary oscillator with an external 12 MHz resonator!

 

Using an external primary oscillator
An external oscillator can be much more accurate than the internal RC oscillator (FRC oscillator). I tried a SiTime SIT2024AA -S8-33E-8.000000E 8 MHz MEMS oscillator connected to the OSC1 clock input. This pin is on i/o pin RA2. Adding the external clock disables RA2 for general i/o. The oscillator worked easily when connected to the SECABB v2 with port expander installed, but the clock signal did not pull down all the way to Vss because of the port expander. This means that on the SECABB you cannot turn on the port expander interrupt INTB because it can affect clock integrity.
Solutions:
-- You could remove the port expander when using the external oscillator, but that is often not feasible.
-- If you do not need i/o expander interrupts, cut pins 19 and 20 (INTA and INTB) off the port expander IC (see datasheet).
-- The full work-around for the pin-use collision is to first turn on the interrupt mirror function on the port expander so that only one interrupt pin is necessary, then cut off port expander pin 19 which is attached to RA2. In the function initPE() you would modify the setting CLEAR_MIRROR to SET_MIRROR, then only detect an interrupt on RA3.

One line of the config_1_3_2.h file (see above and the full list of configuration-bit options) must be changed to:
#pragma config FNOSC = PRIPLL, POSCMOD = EC, OSCIOFNC = OFF
This pragma line:
-- Selects the precision primary oscillator in PLL mode.
-- Enables an external oscillator module input (not a crystal).
-- Turns off the optional function for port OSC2 (RA3), which is to output the peripherial bus clock (PBCLK).
The SiTime oscillator is packaged in SOT23, which is a small, but easily soldered surface mount.
I put it on an Adafruit SOT23 breakout board. You need:
-- to wire a 0.1 uf bypass capacitor between Vdd and GND,
-- connect the OUT to RA2
-- wire the OE input to Vdd.
The oscillator SOT-23-5 package is shown below.

This small circuit has been added toto the PCB for SECABB v2.1. (Look just above the RA4 and RB5 pinouts).
But for testing, a small circuit board was hacked onto the SECABB v2. In the image below,
an existing capacitor (C1) on the board is used as the oscillator power connection and bypass capacitor. The
oscillator output was hard-wired to RA2. You may be able to discern that the SOT-23-5 pin 2 (middle-right side)
on the oscillator package was not soldered. That is because it is internally a no-connect. The left-middle pad
has a droplet of solder on it by mistake, but there is no pin there anyway.

For many applications, the internal oscillator ( fast RC, FRC, 8 MHz ±0.9%) is good enough, but for time keeping or for music synthesis, a 1% error is too high! For time keeping, you need better than 0.005% accuracy (4 seconds/day), and a cheap watch is at least 10 times that accurate! The MEMS oscillator is good to about 0.003%. For music, you need at least 0.2% accuracy. Comparing audio synthesis between the internal oscillator and the external MEMS oscillator for a nominal 440 Hz sine wave gives 441.36 Hz (0.3% error) for the internal oscillator and 440.04 (0.009% error) for the external MEMS oscillator. It turns out that the calculation for 44 KHz sample rate was rounded off from 909.1 cycles (40 MHz/44 KHz) to 909 because the timer compare-register can only be set to integer values. So the frequency measured should be about 0.01% high, which it was. The TDS1012 oscilloscope specfication gives an frequency measurement accuracy of 0.005%, so the MEMS oscillator is approximately as accurate as the scope can measure. Changing the sample rate to be an exact 1000 cycles results in 440.00±0.03 Hz variation in the scope measurement.

Copyright Cornell University October 21, 2019