TL16C754B: Need new chip

Hi Bobby,

Thanks for the info.  I am trying to avoid a PCB re-spin. I am reviewing the spec for TL16C754B (68 pin) version.  The old device has Up to 5 Mbit/s data rate at 5 V and 3.3 V and the TL16C754B has Supports Up To 32-MHz Oscillator Input Clock ( 2 Mbps) for 3.3-V Operation.  Can this be an issue?  can the data rate be adjusted with faster clock? TL16C754BFN

Thanks,

Jegan

Hi Jegan,

I remember working with a UART design engineer about 6 years ago. I think they mentioned the the oscillator circuit had a potential flaw, they were concerned about the XTAL2 output pin and whether it turn on fast enough to keep the clock oscillating at the correct speed. Since the 754B device seems to limit the speed based on the Vcc, I think this means with a lower Vcc value the XTAL2 output pin may not turn on fast enough for higher speed clocks. If your timing circuit needs to use the XTAL2 pin then I would assume the datasheet max value should be followed. (I technically can't tell you to use the device outside of the recommended parameters). 

The datarate follows the clock by 16x. So if you want a baudrate of 1Mbps you need a clock that is 16x faster. Since this is Vcc related as well, I wouldn't try to push it past the datasheet recommended values anyways.

Do you know what the max datarate of your UART device is currently using? Most customers I talk to use 9600bps or 115kbps. It's pretty rare for customers to go over 1Mbps. 

Please also note: The B version of this device also has an errata document. You may need to adjust software based on this errata.

https://www.ti.com/lit/er/sllz048/sllz048.pdf?ts=1718838539519&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTL16C754B

-Bobby

Hi Bobby,

After further review, the speed isn't going to be an issue.  The real issue that I am facing is that the current device uses Motorola 68 mode.  Which has five address lines. TI has parts with three address lines. Do you have any parts with Motorola 68 mode? Is there a circuit that I can add to mimic the three address lines to five?

Thanks,

Jegan

Jegan Easwaramuthali said:

Do you have any parts with Motorola 68 mode? Is there a circuit that I can add to mimic the three address lines to five?

I don't think we have any devices that use motorola mode with 5 address pins.

Externally, I think you may be able to use a 2 to 4 decoder circuit to try to modify the output. The pins that should go into the A3 and A4 would feed into the decoder circuit and then the decoder output circuit would feed into the CSA-CSD pins of the TL16C754B device. You would probably need to also add in some timing delays tin the firmware/software side to clean this up. 

The CS pin for the competitor device isn't present on the TL16C754B. So with our device you would need 4 pins to control (CSA to CSD) while the competitor device uses CS and A3/A4 to make up the other chip select pins. 

Seems like it would be easier to just route new pins from the MCU into the UART device since you would probably still need to modify the software and still do a PCB modification. 

-Bobby