[LPC540xx]: Regarding to SPIFI Clock Frequency

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[LPC540xx]: Regarding to SPIFI Clock Frequency

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[LPC540xx]: Regarding to SPIFI Clock Frequency

The series of LPC540XX chips are flashless, only LPC54018JXM and LPC54S018JXM integrate internal QSPI Flash. The typical part numbers are LPC54018J2(4)M and LPC54S018J2(4)M. Some customers have questions about the concept of SPIFI interface and clock configuration when using this series of chips. This article mainly explains this.

  1. Introduction to SPIFI

SPIFI (SPI Flash Interface) is an SPI Flash interface that can help microcontrollers replace large-size, high-cost parallel Flash with small-size, low-cost serial Flash. Using SPIFI technology, the external serial FLash can be mapped to microcontroller memory to achieve on-chip memory read effect, that is, cost can be optimized and Flash size can be increased while ensuring the operating speed.

The electrical interface of SPIFI is as follows:



In the LPC540XX series of chips, if the part number includes M, QSPI Flash is integrated inside chip; if the part number does not include M, the QSPI Flash is externally connected to chip. The following picture shows the comparison of LPC54S018JXM and LPC54S018 in SPIFI structure:



  1. SPIFI clock frequency description

Taking LPC54S018J4M as an example, the SPIFI clock frequency is described below in the UserManual. SPIFI supports 1/2/4bit transmission mode. In 4bit transmission mode, the maximum transmission rate is SPIFI_CLK/2 bytes per second. The data transmission rate is up to 52Mbytes /s, that is, it takes two clock to transmit one byte. If you want to configure the SPIFI transmission rate to 52Mbytes /s, it needs to be in 4bit mode, so SPIFI_CLK is configured to 104M.





The SPIFI clock source is as follows in LPC54S018J4M Datasheet. By default, the SPIFI clock source is FRO96. For example, when the SPIFI clock is configured to 96M, in 4bit transmission mode, the transmission rate is 96/2=48Mbyte/s.



The LPC54S018J4M uses W25Q32JV-DTR as the internal SPIFI Flash. The figure below shows the maximum clock frequency it supports. In 4bit transmission mode, the maximum transmission rate is 133/2=66.5Mbyte/s, which is greater than SPIFI's maximum transmission rate of 52Mbyte/s. It shows that the maximum data transmission rate of W25Q32JV can meet the requirements of LPC54S018J4M QSPI Flash interface for communication rate.



3Change SPIFI clock frequency

The description of the SPIFI clock frequency in UserManual is as follows. In setup_lpc54s018m.c, the SPIFI clock frequency is defined on the address with an offset of 0X1C (the macro is defined as IMG_BAUDRATE), and the initial value is 0. According to the following table, when IMG_BAUDRATE=0, the SPIFI clock frequency is 24M. Since the default SPIFI clock source is the internal clock FRO96M, the SPIFI clock can be configured up to 96MHz in the following table by modifying the value of IMG_BAUDRATE.



       There are two ways to modify the SPIFI clock.


3.1 Modify the SPIFI clock through IMG_BAUDRATE

Before the main function runs, IMG_BAUDRATE is obtained by BOOT ROM to set the SPIFI clock frequency. If the requirement for the SPIFI clock rate is less than or equal to 96M, it is recommended to directly change the macro definition of IMG_BAUDRATE in setup_lpc54s018m.c to change the SPIFI clock frequency, as follows:



3.2 Modify the SPIFI clock through system config

Another method is to modify the SPIFI clock frequency by changing the SPIFI frequency division coefficient in user code, as follows:



The result is as follows. The SPIFI clock frequency is set to 96M.



If you want to configure a higher SPIFI working clock, such as 104M, you must use a higher frequency external clock source to adjust the PLL coefficient and SPIFI frequency division coefficient in order to achieve the required clock frequency.

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