Visible to Intel only — GUID: exh1481130155791
Ixiasoft
Visible to Intel only — GUID: exh1481130155791
Ixiasoft
16.5.10. Card Read Threshold
When an application needs to perform a single or multiple block read command, the application must set the cardthrctl register with the appropriate card read threshold size in the card read threshold field (cardrdthreshold) and set the cardrdthren bit to 1. This additional information specified in the controller ensures that the controller sends a read command only if there is space equal to the card read threshold available in the RX FIFO buffer. This in turn ensures that the card clock is not stopped in the middle a block of data being transmitted from the card. Set the card read threshold to the block size of the transfer to guarantee there is a minimum of one block size of space in the RX FIFO buffer before the controller enables the card clock. †
The card read threshold is required when the round trip delay is greater than half of sdmmc_clk_divided.†
Bus Speed Modes | Round Trip Delay (Delay_R) 42 | Is Stopping of Card Clock Allowed? | Card Read Threshold Required? |
---|---|---|---|
SDR25 |
Delay_R > 0.5 * (sdmmc_clk/4) Delay_R < 0.5 * (sdmmc_clk/4) |
No Yes |
Yes No |
SDR12 |
Delay_R > 0.5 * (sdmmc_clk/4) Delay_R < 0.5 * (sdmmc_clk/4) |
No Yes |
Yes No |
Section Content
Recommended Usage Guidelines for Card Read Threshold
Card Read Threshold Programming Sequence
Card Read Threshold Programming Examples
Delay_R = Delay_O + tODLY + Delay_I †
Where: †
Delay_O = sdmmc_clk to sdmmc_cclk_out delay (including I/O pin delay) †
Delay_I = Input I/O pin delay + routing delay to the input register †
tODLY = sdmmc_cclk_out to card output delay (varies across card manufactures and speed modes) †