In message , "Graham, Peter" writes >I am having trouble with getting the external series crystal oscillator, >as shown in the PIC16C6X data sheets, running. I have had no trouble in >getting external parallel circuits going on other projects but this is >the first series circuit I have used. I need to use an external series >circuit because the crystal selection (5.0688 MHz) to get the UART >working at the required 9600 baud with no timing errors. > >My problem is that I can only get the oscillator to run at 166KHz. The >only difference in the circuit is that I am using an ALS chip instead of >an AS and I have changed the resistor values to bias the inverters >properly. > >Am I missing something really simple and stupid or can someone please >point me in the right direction. That type of oscillator sometimes works properly and sometimes doesn't, it all depends on the crystal characteristics! I've had problems with them myself. A major UK crystal manufacturer points this out in their data book, and for an oscillator between 4 and 14 MHz recommends this circuit using two inverters: /| |\ _____/ |____________| \___________ | \ | | | / | \| | |/ | | |__RFC __ R __| | | | | |__ TC ___ X _| | | | | C1 C2 | | GND GND TC = 60 pF trimmer C1 = 330 pF C2 = 220 pF RFC = 100 uH R = 430R The above values are for standard TTL, and will need changing for other families. Leon -- Leon Heller: leon@lfheller.demon.co.uk http://www.lfheller.demon.co.uk Amateur Radio Callsign G1HSM Tel: +44 (0) 118 947 1424 See http://www.lfheller.demon.co.uk/dds.htm for details of my AD9850 DDS system. See " "/diy_dsp.htm for a simple DIY DSP ADSP-2104 system.