RTC calibration in LPC54608J512BD208

prasannanaik · ‎03-10-2021

Hi,

Can someone share the process of calibrating RTC of LPC54608J512BD208.

I am using this controller in a Protection Relay wherein timestamp of Fault needs to be shown to user accurately.

Right now the error is very high and due to cost constraints, i can't use less ppm crystal.

Thanks & best regards,

Prasanna

ZhangJennie · ‎03-10-2021

Hi

I'm afraid the LPC546xx has no way to calibrate the RTC. some other parts like LPC17xx has an enhanced RTC which offers a calibration option but LPC546xx doesn't have.

For LPC546xx, 32-bit Real-time clock (RTC) with 1 s resolution running in the always-on power domain. A timer in the RTC can be used for wake-up from all low power modes including deep power-down, with 1 ms resolution.

Have a nice day,

Jun Zhang

prasannanaik · ‎03-11-2021

Hi ZhangJennie,

We are at a product release stage so changing controller would be very difficult.

I have observed error of around 30 sec in one day, what could be reason for this as i am using a 20 ppm crystal (Xc = 6pF) with load capacitances CL1,CL2 = 6pF.

Thanks & best regards,

Prasanna

ZhangJennie · ‎03-12-2021

Please refer our recommendation to design your RTC circuit.

DS:

13.5 RTC oscillator

13.5.1 RTC Printed Circuit Board (PCB) design guidelines

RTC Printed Circuit Board (PCB) design guidelines
• Connect the crystal and external load capacitors on the PCB as close as possible to
the oscillator input and output pins of the chip.
• The length of traces in the oscillation circuit should be as short as possible and must
not cross other signal lines.
• Ensure that the load capacitors CX1, CX2, and CX3, in case of third overtone crystal
usage, have a common ground plane.
• Loops must be made as small as possible to minimize the noise coupled in through
the PCB and to keep the parasitics as small as possible.
• Lay out the ground (GND) pattern under crystal unit.
• Do not lay out other signal lines under crystal unit for multi-layered PCB.

Here is our demo board reference design

Have a nice day

Jun Zhang