Introduction

Some applications require oscillators not already served by the on-chip oscillators inside a GreenPAK IC. This project describes two designs wherein a few passive external components can be used as an oscillator, which is connected to a GreenPAK. The first design is a low-power RC oscillator. The second is a low-power 32 kHz crystal oscillator circuit. In both cases, the GreenPAK design is similar; GPIO pins are used and further internal components are not required. 

Below we described steps needed to understand how the external oscillator devices have been programmed. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the device.

Low Power (sub-µA) RC Oscillator 

An RC oscillator, using external components, allows the user to adjust frequency by adjusting the component values. RC oscillators can easily be made with any GreenPAK chip, but dual-rail chips additionally can make such RC oscillators very low power, down to sub-µA levels, by allowing the use of a resistor to limit the power drawn from the secondary rail. The following design implements an example with the dual-rail SLG46121V, but any dual-rail GreenPAK could be used.

Circuit Design

Fundamentally, a typical oscillator consists of an inverting gain with feedback. In GreenPAK, this can be implemented with just a pin-to-pin connection as shown in Figure 1. Ensuring no other blocks are in the signal path helps to minimize power consumption and latency. The input pin at PIN12 is set to low-voltage digital input (LVDI) mode, which draws relatively little current even with a slow analog signal near its threshold (unlike a normal CMOS input which can have a significant shoot-through current). The PIN12 signal feeds into the OE pin output of PIN10, which is configured as a 3-state output. PIN10’s input is wired to ground. The result is that when OE is high, the output is driven low. When OE is low, the output is disabled allowing it to be pulled high by an external pull-up resistor. Thus, we have the requisite inversion. Functionally this is equivalent to an NMOS, as shown in Figure 2. Externally PIN10 is wired to PIN12, completing the feedback loop. The frequency characteristics of the feedback loop can be controlled by the RC on the wire.

In this example, a separate output (PIN3) was used as a buffered test point to check the frequency without oscilloscope probes loading the feedback loop, which has a high-Z (10 MΩ) state. 

PIN12 and PIN10 are powered by VDD2 (indicated by the yellow color). Externally, VDD2 is connected to VDD by a 10 MΩ resistor to limit the current. PIN10 is pulled up to VDD2 by a 10 MΩ resistor, with a 100 pF capacitor connected to ground. The cycle time can be estimated from the RC decay, i.e. time = RC * ln(VDD/(VDD-Vth)). For example, with C1*(R1+R2) giving a 2 ms time constant, the LVDI input threshold voltage (Vth) of 900 mV, and VDD at 3 V, the cycle time comes out to 713 us, or equivalently,1.4k Hz, roughly matching up with the measured results shown in Table 1.

Results

Table 1 shows ISUPPLY and frequency measurements of the SLG46121V external RC oscillator circuit, compared to the internal oscillator of the SLG46620V. The SLG46620 low-frequency internal oscillator was chosen because it has one of the lowest power internal oscillators of the various GreenPAK chips. The supply current of the SLG46121V without the buffered output test point is also shown since the output buffer consumes some switching power. Note the SLG46121V itself does not have a low-frequency internal oscillator that can run at power as low as the SLG46620; its lowest power internal oscillator can run at about 5 µA, like many other GreenPAK chips. Other aspects to note are the effects of VDD on the frequency and supply current. Figure 5 graphs supply current...

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