- Sun Jun 01, 2008 8:53 pm
#49403
I am using an MSP430F1611 and the 32.768 KHz crystal oscillator is running slowly enough that it loses about 1 second per minute. I arrived at this by comparing the time reported by FORTH Inc's TIME function included with SwiftX and the time displayed on my PC. Unfortunately, I don't have an oscilloscope or a counter-timer.
I thought this might be due to stray capacitances on my PCB so I tried it using another 430F1611 in the socket on my TI FET430P140 evaluation board with the crystal supplied by FORTH Inc and it is even worse, losing about 2 seconds per minute.
The processor on my board is running from a supply voltage of 3.3V and the tracks (0.2mm wide) to the crystal are 1.0 and 1.5 cm long. The board is four layer with buried power and ground. The crystal is a Citizen CM415 32.768 KHz surface mount part and is specified for use with 12.5 pF capacitors such as are internal to the MSP430 series. I calculate the capacitance to ground of the crystal's solder pads to be 0.026 pF and the tracks would add a similar amount - certainly small in comparison to the 12 pF in the processor.
The processor on the FET430P140 is running from 2.75 Volts and the crystal is a leaded cylindrical can on which I don't see any markings.
In either case this is pretty horrible performance from a crystal oscillator and is certainly of absolutely no use for time keeping.
Has anyone else encountered this problem, and better yet solved it? One thought I have is that the crystal might be driven too hard. Would some series resistance be worth trying? Are there any oscillator experts out there?
Bob
I thought this might be due to stray capacitances on my PCB so I tried it using another 430F1611 in the socket on my TI FET430P140 evaluation board with the crystal supplied by FORTH Inc and it is even worse, losing about 2 seconds per minute.
The processor on my board is running from a supply voltage of 3.3V and the tracks (0.2mm wide) to the crystal are 1.0 and 1.5 cm long. The board is four layer with buried power and ground. The crystal is a Citizen CM415 32.768 KHz surface mount part and is specified for use with 12.5 pF capacitors such as are internal to the MSP430 series. I calculate the capacitance to ground of the crystal's solder pads to be 0.026 pF and the tracks would add a similar amount - certainly small in comparison to the 12 pF in the processor.
The processor on the FET430P140 is running from 2.75 Volts and the crystal is a leaded cylindrical can on which I don't see any markings.
In either case this is pretty horrible performance from a crystal oscillator and is certainly of absolutely no use for time keeping.
Has anyone else encountered this problem, and better yet solved it? One thought I have is that the crystal might be driven too hard. Would some series resistance be worth trying? Are there any oscillator experts out there?
Bob