running 2 PIC chips off same xtal

[Build-A-Burger]

I'm building a circuit that uses a PIC16F877A and also a PIC16F84. Would there be any problem in running both of these off the same 20Mhz crystal and 22pf caps?

 

[bigdogguru]

I've used a crystal "can" oscillator (XO) to drive several microcontrollers in the the past.

I would recommend implementing a simple external oscillator using your crystal and an inverter from which to drive the PICs, rather than try and utilize the clock signal directly from the first PIC to drive the second PIC.

Stray capacitance could certainly be an issue with the latter method.

As per Microchip's recommendation,

Reference: PICmicro Mid-Range MCU Family Reference, Section: 2.3.5 External Crystal Oscillator Circuit for Device Clock, Page: 2-11

2.3.5 External Crystal Oscillator Circuit for Device Clock

Sometimes more than one device needs to be clocked from a single crystal. Since Microchip
does not recommend connecting other logic to the PICmicro’s internal oscillator circuit, an exter-
nal crystal oscillator circuit is recommended. Each device will then have an external clock source,
and the number of devices that can be driven will depend on the buffer drive capability. This circuit
is also useful when more than one device (PICmicro) needs to operate synchronously to each
other.

Either a prepackaged oscillator can be used or a simple oscillator circuit with TTL gates can be
built. Prepackaged oscillators provide a wide operating range and better stability. A well-designed
crystal oscillator will provide good performance with TTL gates. Two types of crystal oscillator cir-
cuits can be used; one with series resonance, or one with parallel resonance.

Figure 2-4 shows implementation of an external parallel resonant oscillator circuit. The circuit is
designed to use the fundamental frequency of the crystal. The 74AS04 inverter performs the
180-degree phase shift that a parallel oscillator requires. The 4.7 kΩresistor provides the
negative feedback for stability. The 10 kΩ potentiometer biases the 74AS04 in the linear region

Figure 2-5 shows an external series resonant oscillator circuit. This circuit is also designed to use
the fundamental frequency of the crystal. The inverter performs a 180-degree phase shift in a
series resonant oscillator circuit. The 330 kΩ resistors provide the negative feedback to bias the
inverters in their linear region.

When the device is clocked from an external clock source (as in Figure 2-4 or Figure 2-5) then
the microcontroller’s oscillator must be configured for LP, XT or HS mode ( Figure 2-3).

BigDog

 

[Build-A-Burger]

This is so cool - thanks a lot. I was going to use the 2nd PIC1684a with the PIC16877a and the parallel port with a DS1220Y 2k ram chip to give the 877a more ram. Since the parallel port can only be controlled by an external source, the PIC1684 will control the access to the ram chip and the PSP on the 877a. If both chips are running at the same clock speed, then I can expect when the PIC84 will read/write the PSP after the 877a sends a signal to the PIC84 to start the sequence.
This design is the topic for another post, but if anyone would like to comment on it, you can see it at **broken link removed**. Thanks again.

 

[pasicr]

Excellent explanation BigDog,
very usefull info,
regards