Цитата(Палыч @ Mar 17 2016, 10:00)
В AVR активный сигнал на входе SS (на ноге SS, настроенной как ввод) разрешает работу сдвигового регистра SPI. И не важно в каком режиме работает SPI - master или slave. Для мастера низкий (активный) сигнал на входе SS фактически означает готовность подчиненного устройства.
1. Сигналом SS управляет мастер.
2. Если сигналом SS управляет не мастер, см. п.1.
Цитата(ATMega 328p datasheet)
19.3.2 Master Mode
When the SPI is configured as a Master (MSTR in SPCR is set), the user can determine the
direction of the SS pin.
If SS is configured as an output, the pin is a general output pin which does not affect the SPI
system. Typically, the pin will be driving the SS pin of the SPI Slave.
If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin
is driven low by peripheral circuitry when the SPI is configured as a Master with the SS pin
defined as an input, the SPI system interprets this as another master selecting the SPI as a
slave and starting to send data to it. To avoid bus contention, the SPI system takes the following
actions:
1. The MSTR bit in SPCR is cleared and the SPI system becomes a Slave. As a result of
the SPI becoming a Slave, the MOSI and SCK pins become inputs.
2. The SPIF Flag in SPSR is set, and if the SPI interrupt is enabled, and the I-bit in SREG is
set, the interrupt routine will be executed.
Thus, when interrupt-driven SPI transmission is used in Master mode, and there exists a possibility
that SS is driven low, the interrupt should always check that the MSTR bit is still set. If the
MSTR bit has been cleared by a slave select, it must be set by the user to re-enable SPI Master
mode.
Связь двух контроллеров по SPI.
Mar 17 2016, 07:36
