Atmega8 Sleep mode doesn't work with USART

If you go to sleep mode, something has to happen to make the AVR wake up from sleep mode. This is usually a reset or an interrupt. In your code, you don't use interrupts on the UART, so when you enter sleep mode, the AVR stays there.

Meindert

Thank you Meindert Sprang.

But It's a little bit different. In my case, I fix code like below.

void main(void) { Hardware_Init();

while(1) { Uart_Put_Char('A'); __asm volatile ("sleep"); Uart_Put_Char('B'); } }

and My system doesn't print even "A".

I think something wrong. Do you know why ?

That is most likely because you put the AVR to sleep before the "A" has left the UART. Your Uart_Put_Char() first checks the UDRE bit, then it loads a new byte in the UDR register. If you then call "sleep", the AVR goes to sleep before the UART has finished shifting the byte out. Checking for UDRE

*after* the write to UDR doesn't help either because the UART is double buffered. UDRE flags the state of the UDR register and not the shift register. To test this, insert

while ( !( UCSRA & (1

If his uart transmit routine were interrupt driven then the avr might go to sleep and wake up in the transmit routine. Note that there are different sleep modes you can specify, and some of them will NOT allow the uart to run, there is a mode that WILL allow the uart to receive data while the cpu is asleep and you need to use that mode to wake up on a character received. Of course the power savings isn't as great with this mode.....

BTW, I'm working on a project with a mega128 avr using external ram. In putting this one to sleep I first disabled external memory mode so the chip select of the ram (wired to A15) would go high to save the current pulled by the selected ram chip (dropped about

30ma!). But to do this I first had to relocate the stack to internal ram (I'll leave the reason for this as an excerise for the reader!). The stack is restored after the cpu wakes up. The cpu is put to sleep in mainline code, ANY interrupt will wake up the cpu, and after returning from interrupt we are at the instruction AFTER the sleep instruction. NOTE that the interrupt code must NOT access any data in external ram! (think of all the fun I am having with this!)

You have to wait for the characyter to lkeave the uart --- then goto sleep. I use ATMEGA128 with UART and also with external 32KHz xtal on timer 0 for power save modes. So far I'm down to 35uA. Be careful with external pull-ups, make sure ports are configured for input lest the chew up power while in sleep. Shut everything else off --- including JTAG debug interface, analog comparator, and watchdog timer (can be done in software), otherwise you will not get anywhere near

10's of uA.

David Howard

left

a

UDRE