Raghu, you want an expert reply, so here is a reply directly from Cygnal.:

Question:
In our applications, we need to have a good time base for two reasons: 1) the application requires time-related activities; and," 2) serial communication at industry standard baud rates (9600, 19200,...). Will it be necessary to provide an external crystal in order to obtain accurate bit-rate serial communication? Could we use a (quick) calibration method during manufacturing that would calibrate the internal high frequency oscillator, store the calibration parameter in FLASH, and base serial baud rates and real-time on the calibration parameter? Is the internal high frequency oscillator consistent over many days/months/years?

Answer:
1. The absolute accuracy of the current internal oscillator is on the order of about +/- 20% from part to part. It is sensitive to changes in power supply voltage, about 6.5%/V, and almost immune to temp variations (<1% over 40 to +85 degrees C). Aging, that is, the change in oscillator frequency over months and years, is not significant.

2. Regarding calibrating the internal oscillator during manufacturing, this is possible, and is not too hard to implement. The important parameter to control is power supply voltage. It is vital that the power supply voltage used in calibration is the same value as that used in actual operation. It's really easy to configure a timer using SYSCLK as the timebase to measure the high time of an external signal of 'known' frequency (or duration, let's say 1ms). Storing this value in FLASH from code is also easy.

3. Regarding using the internal oscillator to drive the UART clock, one of the best ways of doing this is to implement an auto-baud detect feature. If your SYSCLK frequency is substantially higher than your bit rate, it's possible to measure an incoming UART bit period accurately enough to set up your local UART to retransmit at the received baud rate. As a rule of thumb, SYSCLK should be at least 750 * the incoming bit rate (for SYSCLK = 11.0592 MHz, the max bit rate is 14.4Kbps; for SYSCLK = 18.432 MHz max bit rate is 24Kbps). We will have an appnote describing a method for this in the next few weeks.

Date: 09/22/2000

(above text is from Cygnal, not me)

- Lee