Achieving < 1% Precision Clocking Solution with External-R under Practical Constraints

Aniruddha Roy, Preetham N. Reddy, Nitin Agarwal, Nikhil Das

Abstract: An external resistor <tex>$(R_{EXT})$</tex> based clocking solution with <tex>$&lt; 1\%$</tex> precision is presented in this paper. Though <tex>$R_{EXT}$</tex> can have much better temperature coefficient (TC), e.g. TCl <tex>$=25\text{ppm}/^{\circ}\mathrm{C}$</tex> and negligible TC2, practical considerations in an integrated system-on-chip (SoC), such as micro-controller (MCU) opposes the benefits. For low cost SoCs, it is desired that the EXTR-pin (i.e. the pin where <tex>$R_{EXT}$</tex> is mounted) shares a general purpose I/O pin (GPIO). Under this constraint, adjacent GPIO switching can cause significant coupling onto EXTR-pin resulting in intolerable jitter. Additionally, the shared, tri-stated GPIO adds leakage current with nonlinear temperature profile, into <tex>$R_{EXT}$</tex> causing curvature error in frequency. These are solved by novel circuit solutions which finally help achieve the targeted precision. Measured and simulated results are provided to support the claims.