Recently, a student asked me: “How many SPI chips can I connect to my 51 microcontroller board? Is it not mentioned in the chip manual, or did I miss it? Moreover, I want to connect a screen, a memory card, a temperature sensor, and a wireless module.” Today, let’s discuss this topic.
First, let’s talk about how SPI works.
SPI communication generally requires 4 lines:
SCLK: Clock line, generated by the master, which is the 51 microcontroller, and all devices follow its rhythm.
MOSI: Master sends data, slave receives data line. The master uses this line to communicate.
MISO: Master receives, slave sends data line. The slave uses this line to respond.
CS, SS: Chip select line, which is the most critical line. One CS line corresponds to one slave device.
The essence of SPI lies in its chip select CS. When the master wants to communicate with a specific slave, it pulls down the corresponding CS line, just like calling attendance in school: “Student A, it’s your turn now.” At this time, all other slaves with CS lines at a high level will automatically ignore the signals on the MOSI and SCLK lines. Although they can hear the sounds on the bus, they will not respond; only the one being called will answer.
So, how many can be connected?
The answer lies in the chip select CS line.
The number of available IO ports on the 51 microcontroller theoretically determines how many SPI slave devices can be controlled.
For example, if your 51 microcontroller, such as the STC89C52, has 32 IO ports, aside from the 3 lines used for basic SPI communication (SCLK, MOSI, MISO), you can free up 10 IO ports specifically for chip select lines. Thus, you can connect a maximum of 10 SPI devices.
Another advanced method, if you find IO ports too precious, is to use external chips, such as I2C to IO port expansion chips like PCA9535 or decoders like 74HC138. You can control this expansion chip with one or two IO ports, and this expansion chip can generate 8 or even 16 independent chip select signals. This way, you only use 2 IO ports to indirectly control 8 SPI devices, greatly saving the master IO resources.
Therefore, theoretically, by using cascading expansion chips, you can connect dozens or even hundreds of devices, which is what is referred to as almost unlimited.
What practical limitations should be considered in development?
Although theoretically many can be connected, in actual project implementation, one cannot only look at theory. You also need to consider the following:
IO port resources: This is the most direct limitation. The IO ports of the 51 microcontroller are already tight, and you also need to drive LEDs, buttons, displays, etc. The number of ports allocated for SPI chip select determines your upper limit.
Software complexity: Each additional device requires the program to manage an additional chip select signal. The more devices there are, the more complex the program logic becomes, and the higher the probability of errors.
Bus load capacity: SPI bus generally operates at a higher frequency. The more devices connected, the greater the capacitance on the line, which can degrade the signal waveform and increase communication errors. If there are many devices and long wiring, you may need to consider adding a bus driver, such as 74HC245, to enhance the signal.
Power load: Each peripheral chip consumes power, and you must ensure that the microcontroller system’s power supply can support all devices working simultaneously; otherwise, voltage instability may cause the system to restart repeatedly.
My suggestion is that for general development, connecting 2-4 devices is usually the most practical and stable. If you need to connect more devices, consider using IO expansion chips, such as 74HC595, to increase chip select lines, or use multiplexers to share chip select lines. It is essential to pay attention to the bus load capacity, as too many devices may lead to signal quality degradation. Of course, you can also choose microcontroller models with more IO pins.
The 51 microcontroller can connect many SPI peripherals through reasonable hardware design, but quality is more important than quantity.