Dapu Telecom GPS Clock Module CM55F-K129 - Page 1
Forum section: electronics / general

author: Birb
author_rank: Contributor
timestamp: July 17, 2025, 06:34:17 am
content: While taking a break from voltage references, I decided to mess around with horological devices.Aside from the well known CTI OC5SC25 / OSC5A2B02, I found an extracted CM55F-K129 at approximately 12USD. It appears that there is no prior mention of this device here.Essentially, it consists of an OCXO, an MCU, a DAC and an FPGA. The datasheet (attached) claims that it is able to receive a 1PPS signal from a GPS and auto-tune itself to compensate. (Something about an adaptive model to compensate for drift?). As such, the claim is that in a free-running state, it achieves 1.5us/d drift. (After 7 days running + 3 days GPS lock). Unfortunately, it appears that this stability is only over a short period (expected due to the nature of this device), perhaps for one day or more.The datasheet for the CM65A-D129 (related device) does list a table attempting to explain the training time (time with GPS PPS reference) and the holdover capability, but it's hard to follow (page 12 of second datasheet).Whilst it lacks in analog control, there might be some method to digitally tune the DAC value. The datasheet, however, only lists the output test message, and information on how to configure the DAC is absent.For mine, the output was:Code:$PDP,00,0,F,Q,-0000,33742.0000,32769.0000,000,000,00000.0000,00000.0000,00000.0000,00000.0000,0-00,+000.0000,-0000,www.dptel.com,1.1,2011-05-16*55This is essentially identical to the default test message, with the only exception being the PWM DAC values being different (33742 and 32769 as opposed to 32768 and 32768).DAPU seems to not be providing datasheets as all the datasheets I've found so far are scattered across the web and seem to be outdated. In their Product Portfolio, the CM55F-K129 is specified as a Stratum 1 Time Server Clock Module, so there's that. (Attachment 3)I'd quite like to take it apart to inspect the architecture (maybe replace the LDO with another VREF?), but I only have one and I might save it for something useful (High Stability 10MHz GPS Ref?). I also don't have the equipment to test its rated accuracy and drift characteristics, but cross testing with other OCXOs shows that it agrees well.Does anyone have any related information about or experience utilizing this module?Also, is it worthy of being used as a frequency reference (even a in free running state?) (or maybe as a GPSDO?)Addendum: The link where I originally obtained it shows it out of stock now. It is sometimes available on Taobao and ebay, with the former offering it at lower prices.Any help is appreciated, thanks a lot.

author: XLB
author_rank: Newbie
timestamp: August 28, 2025, 08:39:12 am
content: Hello, could you tell me if you have the data sheet for the CTI OC5SC25 series? I have been unable to locate it so far, and it would be greatly appreciated if you could provide it.

author: lugaw
author_rank: Regular Contributor
timestamp: August 28, 2025, 02:45:43 pm
content: Seems like a complete system, you just need to supply a 1pps reference from a GPSDO and it will adjust the internal oscillator.

author: ru_tash
author_rank: Regular Contributor
timestamp: September 06, 2025, 10:11:53 am
content: Quote from: Birb on July 17, 2025, 06:34:17 amrecently I've received same module, I tested it, connected to 1 PPS, after few minutes it's got locked.it seems like normal unit, like any other Chinese OCXO.