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ST10X167/F168
Minimizing Power Consumption for SPI EEPROMs
APPLICATION NOTE
Many industrial and automotive applications use external SPI EEPROMs to back-up key data. Power consumption must be minimized during back-up to SPI EEPROMs. This becomes critical when decoupling capacitors are used to store the energy necessary to complete the data back-up process. This application note explains how to use SPI EEPROM Page Mode Operation for low power consumption, and gives advice on optimum data mapping and peripheral configurations. A C-language implementation of the SPI EEPROM Page Mode operation is contained in the appendix. Configurations outlined in this application note keep the ST10 in idle mode for most of the write to the external EEPROM. A 50% savings in power consumption can be achieved for both ST10F168 and ST10C167 by using SPI EEPROM Page Mode, mapping data into internal RAM and stopping unused peripherals during data back-up. Power consumption savings are defined by the ratio: Idle current / Operating current. To assess the saving for your application, check the power consumption defined in the product Data Sheet. Features such as, built-in error detection on SPI, and EEPROM write protection mechanism, can be used to further secure the back-up of key data into external SPI EEPROM.
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*
Introduction
Power down mode: the CPU and all the peripherals are stopped. Exit from power-down is only possible through an external interrupt signal. This mode gives the best power consumption savings. Idle mode: the CPU is stopped but all the peripherals remain active. Exit from idle mode is through the external interrupt signal or by internal interrupt.
The ST10 microcontroller has 2 power reduction modes:
*
For most applications, idle mode is preferred because the device can resume operation on a pre-defined peripheral event (end of transfer on SPI bus and/or end of write into external EEPROM). Program flows should take advantage of idle mode by relying on peripheral's hardware features rather than software instructions.
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2.1
Minimizing power consumption
SPI EEPROM Page Mode operation for low power consumption
Many SPI EEPROMs operate read and write sequences in Page Mode (see Non-Volatile Data Books on http://www.st.com). This feature can be used to reduce ST10 power consumption during data transfer. The program flow described below and shown in Figure 1, transfers data to an SPI EEPROM but keeps the ST10 in idle mode for most of the transfer.
Program flow
1 2 3 4 Data is transferred from the RAM to the SPI transmit register by PECC transfer. The ST10 is put in idle mode during page transfer to the external SPI EEPROM. When the page transfer is complete, the CPU is put in idle mode for the duration of the write to SPI EEPROM (using an internal timer). At the end of the write sequence, the timer wakes-up the ST10 and the CPU checks whether another page is to be written to the external SPI EEPROM.
A C-language implementation and assembly implementation of the power control instructions, are given in sections 2.3 and 2.4. A C-routine is given in Section "Appendix - C function example" on page 5.
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Initiate SSC in SPI, Initiate PECC, Initiate timer
Initiate transfer for first page
ST10 in idle mode Interrupt: PECC counter reaches 0
Initiate PEC for next page
Interrupt: end of timer (write duration) Yes
Start timer for EEPROM write
More data to save?
No
End of transfer
Figure 1 Program flow
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2.2
Other ways to reduce power consumption
Mapping data into internal RAM only
Where possible, map data into ST10 internal RAM instead of external memory. The internal RAM can either be the internal dual-port RAM, or the internal XRAM. The current required to address external memory is greater than that for internal memory.
Stop unused peripherals during data back-up
During the data back-up process, stop all unused peripherals in the following ways:
* * * *
Unused timers: clear bit in the relevant TxCON registers, Unused CAPCOMs: stop the CAPCOM timers/counters by clearing bits TxR in T01CON or T78CON registers, Unused PWMs: stop the timers by clearing bit PTRx in PWMCONx registers. Asynchronous serial interface: the baud ASC baud rate generator can be stopped by clearing bit S0R in S0CON register.
Port configuration
Where possible, configure ports for minimum power consumption.
2.3
C language implementation
Power control instructions in C are usually handled by intrinsic functions. For the Tasking tool chain, the intrinsic function for idle is "_idle". The Tasking tool chain can directly handle SFRs provided that the targeted ST10 register definition is included in the C source file. As the ST10 resumes the program just after the "idle" instruction, the interrupt service routines for both the timer and SPI_transmit can be reduced to "RETI". Then the whole program flow can be implemented in one C function. An example of such a C function is shown in Section . This example gives the ST10 configuration for the data back-up process. The program flow can be modified perform a data integrity check, or enable ST10 error detection on SPI (phase error, receive error)
2.4
Assembly implementation
The instruction to put the ST10 in idle is "IDLE". The suggestions made for the interrupt service routines are also applicable here.
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Conclusion
A 50% savings in current consumption can be achieved for both ST10F168 and ST10C167 by using SPI EEPROM Page Mode, mapping data into internal RAM and stopping unused peripherals during data back-up. Power consumption savings are defined by the ratio: Idle current / Operating current. Features such as, built-in error detection on SPI, and EEPROM write protection mechanism, can be used to further secure the back-up of key data into external SPI EEPROM.
Appendix - C function example
The C routines below illustrates an implementation of the program flow detailed in Section 2.1.
// include to have declaration of SRFregisters and SFRbits #include
// function prototypes void init_spi_page_write(); void spi_save(int addr); void spi_send_write_cmd(int addr);
// functions
// main SPI transfer function void spi_save(int addr) { int i; init_spi_page_write(); // init SSC in SPI for page transfer // assume write into EEPROM is enabled
for (i=0; i=3; i++) { SSCIE = 0; spi_send_write_cmd(int addr); SSCTIC= SSCTIC | 0x00C0; _idle; TxR=1; _idle; PECC0 = 0x01yy; } }
// example for fixed length transfer : 4 pages
// mask SPI interrupt during instruction + address phase // send the write command : instruction + address // set Interrupt request to start enable interrupt page transfer +
// put the ST10 in idle until page transfer is completed // set bit TxR to start timer // put the ST10 in idle for the write duration // prepare PECC0 for next page transfer}
// EEPROM is assumed to be reprotected after this
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// function called by main SPI transfer // other functions void spi_send_write_cmd (int addr) { if (addr > 255) { SSCTB = 0x000A; addr = addr -256; } else { SSCTB = 0x0002; } SSCTB = (char) addr; while (SSCTIC & 0x0040 ) { } } // take only low byte // wait end of transmit of 2nd byte // depend on EEPROM size // EEPROM is assumed in write enable state // send command and address for write
// function called by main SPI transfer function void init_spi_page_write() { P3 = P3 | 0x2200; DP3 = PP3 | 0x2200; SSCBR = SPI_reload; PECC0 = 0x01YY; SSCTIC = 0x0078; DSTP0 = 0xF0B0; SRCP0 = 0xE000; SSCCON = 0xC037; Tx = timer_x_reload; TxCON = 0x0; } // SSCTB // buffer is in XRAM starting at E000h // MSB first, ignore errors (check your configuration) // depend on EEPROM write timings // Timer in timer mode, count-up (check prescaler factor) // depend on bit rate // byte transfers, yy is the EEPROM page size in bytes // init SPI to transfer pages
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