STM32 and STM8 devices have some types:
- Low density devices
- Medium density devices
- High density devices
About what "density" mean? About quantity of periphery or about quantity of flash or something else?
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They're arbitrary terms used by ST to distinguish different lines of MCUs within a series. Generally speaking, all of the parts within a line will have all the same peripherals and features; they will differ only in the amount of RAM and flash memory they have, and in packaging.
For instance, here's how ST breaks down the STM32F103 family:
The terms "value line" and "connectivity line" also appear in some series; those are also treated as lines.
About the reference guide of STM32F103 Glossary :
• Low-density devices are STM32F101xx, STM32F102xx and STM32F103xx microcontrollers where the Flash memory density ranges between 16 and 32 Kbytes.
• Medium-density devices are STM32F101xx, STM32F102xx and STM32F103xx microcontrollers where the Flash memory density ranges between 64 and 128 Kbytes.
• High-density devices are STM32F101xx and STM32F103xx microcontrollers where the Flash memory density ranges between 256 and 512 Kbytes.
• XL-density devices are STM32F101xx and STM32F103xx microcontrollers where the Flash memory density ranges between 768 Kbytes and 1 Mbyte.
• Connectivity line devices are STM32F105xx and STM32F107xx microcontrollers.
• Word: data of 32-bit length.
• Half-word: data of 16-bit length.
• Byte: data of 8-bit length.
First let’s explain what Low Density and High Density refers to. Using 256MB modules as an example… you would think the 16 chip module would be called the High Density module and the 8 chip version would be the Low Density module… but it’s backwards, isn’t it? The problem is – the density isn’t referring to the number of chips on the module, it refers to the density of the memory units inside the individual chips on the module.
When people talk about a Low Density module, they’re incorrectly abbreviating their speech, and it’s misleading. What they really mean is – it’s a module with Low Density chips. Think of two pepperoni pizzas. On one pizza each slice has 2 pieces of pepperoni. On the other each slice has 4 pieces of pepperoni. The one with 2 pieces per slice is the low density pizza, and the one with 4 pieces per slice is the high density pizza.
This is what it’s like on the RAM modules. The High Density module only needs half as many chips per side because there are more memory slots inside those chips. If you want Low Density chips, you need twice as many, and so modules with Low Density chips cost twice as much. (The labor to build the module from the assembled parts is about the same, regardless.)