UART RX in low power mode

ktallevi · Joined: 17 Dec 2005 Posts: 58

I am having a rather odd issue. I have a 38.4Khz crystal connected to Timer1. I switch from the main 48Mhz osc to the timer1 crystal to save power. A character coming in on the UART will wake up the PIC.

After programming the PIC or powering it up for the first time, the character is always detected incorrectly. Upon resetting the PIC, the character is detected correctly everytime there after.

Compiler Version: 4.080
PIC18F67J50

Relavent Code Below
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Ttelmah · Guest

The first received character on wake up, normally _will_ be received incorrectly. This is inherent in using the wake up function. It behaves as a way to wake, rather than an easy way to actually receive data. The easiest way, is to send a 'sacrificial' character to wake the chip, then wait for the main oscillator to start, and the chip to rouse, then start sending the real data.
You can have a character received correctly, if you use a new clock, and #use RS232 statement, with the value setup for the correct divisor for the low frequency clock, and select this to receive the first character. To do this, you also need to delay switching to the main clock, till the UART is idle (look at the RCIDL bit). You can also 'cheat' this, by using the set_uart speed function, and selecting as your required baud rate, a value of your real baud rate, *48000000/38400. Again though, you will need to check the RCIDL bit before selecting the real clock, and switching the baud rate to normal operation.
The problem is that the UART is clocked off the slower clock, and therefore will sample at the wrong time...

Best Wishes

ktallevi · Joined: 17 Dec 2005 Posts: 58

Ttelmah, as you can see in my code, I am not putting the PIC to sleep, just switching to the Timer1 clock.

I am using a hardware UART, and the timer1 clock is 4 times the 9600 bitrate. So I should be able to read the character successfully without any error in the 38.4Khz mode.

From what I can tell, my code follows exactly what you recommended for reading the character without error.

Also as I had mentioned, it only fails when I initially program the PIC or power it on, once I issue a reset it reads the character correctly.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

Please read the processor manual thorougly. No standard UART is operating with factor 4 oversampling, most use factor 16. You can't receive 9600 baud with a 38k4 crystal.

It can be seen from your code, how the UART baud generator is actually programmed, cause the respective statements (e. g. #use delay or manual override of baud rate) are missing.